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Wr ^
i r-N
AWV
THE CONTEMPORARY SCIENCE SERIES.
Edited by IIAVELOCK ELLIS.
THE EVOLUTION OF SEX.
THE
Evolution of Sex.
BY
Professors PATRICK GEDDES
AND
J. ARTHUR THOMSON.
IVITff 92 ILLUSTRATIONS.
REVISED EDITION.
THE WALTER SCOTT PUBLFSHING CO, LTD.,
PATERNOSTER SQUARE, LONDON, E.C
CHARLES SCRIBNER'S SONS,
153-157 FIFTH AVENUE, NEW YORIC
190A
HARVARD MEDICAL LIBRARY
HITNE
FRANCIS A. COUNTWAY
LIBRARY OF MEDICiNF
PREFACE
TO THE SECOND EDITION.
THE rapid exhaustion of the large first edition (1889) of
this book, and of subsequent reprints also, seems to
show the need of a general survey of the essential phenomena
of reproduction and sex. That this is in our case associated
with a particular theory of the nature of these, need not hinder
the reader from discriminating between facts and interpreta-
tions. Hence in this revised edition, though many alterations
and additions have been made, the original character of the
work has been retained, and that notwithstanding the difficulty
that the authors have in the past ten years been diverging
biologically — the one towards a " Neo-Lamarckian " position,
the other towards a " Neo-Darwinian " one. Yet they remain
agreed on the main endeavour of the book, which is to set
forth the fundamental unity underlying the Protean phenomena
of sex and reproduction.
The biological interpretation worked out in this volume
may be summarised in three propositions : —
(A.) In all living creatures there are two great lines of
variation, primarily determined by the very nature of proto-
plasmic change (metabolism); for the ratio of the constructive
(anabolic) changes to the disruptive (katabolic) ones, that is of
income to outlay, of gains to losses, is a variable one. In one
VI PREFACE.
sex, the female, the balance of debtor and creditor is the more
favourable one; the anabolic processes tend to preponderate,
and this profit may be at first devoted to growth, but later to-
wards offspring, of which she hence can afford to bear the larger
share. To put it more precisely, the life-ratio of anabolic to
katabolic changes, |^, in the female is normally greater than the
corresponding life-ratio, t, in the male. This, for us, is the
fundamental, the physiological, the constitutional difference
between the sexes; and it becomes expressed from the very
outset in the contrast between their essential reproductive
elements, and may be traced on into the more superficial
secondary sexual characters.
(B.) There is much experimental evidence to show that
the determining stimuli which cause the balance of life to
swing to one side or other, which shunt the plastic organism
to this side or to that, are to be found, at least very largely, in
the external or environmental conditions — food, temperature,
light, chemical media, and so on ; so that the determination of
sex becomes to some extent practicable in an increasing
number of forms.
(C) Yet, the sexual dimorphism, in the main and in detail,
has an adaptive significance also, securing the advantages of
cross-fertilisation and the like, and is therefore to some extent
the result of the continual action of natural selection, though
this may of course check variation in one form as well as
favour it in another.
So far our main theses. Yet these are obviously but the
preliminaries to others. That such an interpretation of the
phenomena of sex and reproduction must have its bearing on
the whole treatment of the problem of the origin of species is
manifest, and it appears to us that these two main variational
PREFACE. Vli
lines, upon which we seek to explain the divergent evolution of
the sexes, are to be detected also in the variety and the species,
in the group and beyond, it may be in the very contrast of
plant and animal. But each new mode of interpretation in-
evitably invites us towards the re-examination of the whole
evolutionary process in all its aspects, in all its products.
And if, as none now deny, the study of the simpler manifesta-
tions of life be a legitimate and even a necessary preliminary
to the understanding of human life itself, we must seek fully
to rise from our elementary interpretation of reproduction and
sex to the study of their vast complexity upon the human
plane — anthropological and social, psychological or ethical,
educational or practical. We have attempted to indicate some
of these considerations more fully in the concluding chapter,
as also in some of our separate papers and encyclopaedia
articles cited in the text, to which a single other reference
may be added.* Thus our volume has been from its incep-
tion a dozen years ago but the beginning of a larger scheme
which its general vastness on one hand, and the pressure of
individual duties and cares upon the other, have alike delayed.
But though this can never adequately be carried out, we still
hope we may be able to do something, together or separately.
The past reception of the book in biological circles has
been varied. It has been severely let alone by some authors,
even when discussing the subject, and generously borrowed
from by others, with or without acknowledgment. It has
sometimes completely failed to be understood ; we think
we may say chiefly by those too pure morphologists to whom
our starting-point, the ordinary physiological conception of
protoplasmic metabolism as laid down by Claude Bernard
• Cf. the authors " On ihe Moral Evolution of Sex," The Ever-
GREEN, Edinburgh, Summer, 1896.
Vin PREFACE.
and his successors, had not been previously familiar. Some-
times too it has been thoughtfully and keenly criticised, and
from this we trust we have not wholly failed to profit But
it has also provoked a certain amount of research, which is
better than all compliments and criticisms. We venture to
think that the general tendency of research has been sub-
stantially to confirm, not weaken our general theory : yet our
hope is that the growing strength of the still young school of
experimental evolutionists may before many years yield results
which will involve not merely a revision, but a recasting of our
book.
From a wide circle, beyond that of professed biologists, we
have also received criticisms suggestive as well as encouraging,
and we trust, therefore, that, with all its shortcomings, this
revised edition will be found freed of at least some errors, and
freshened by the incorporation of some new observations and
thoughts.
P. G.
J. A. T.
Edinburgh, *) ^
Absrdkkn, J
PREFACE
TO THE FIRST EDITION (1889).
IN course of the preparation of critical summaries, such as the
articles "Reproduction" or '"Sex," contributed by one of
us to the " Encyclopaedia Britannica," or the account of recent
progress annually prepared for the Zoological Record by the
other, we have not only naturally accumulated considerable
material towards a general theory of the subject, but have come
to take up an altered and unconventional view upon the general
questions of biology, particularly upon that of the factors of
organic evolution. Hence this little book has the difficult task
of inviting the criticism of the biological student, although
primarily addressing itself to the general reader or beginner.
The specialist therefore must not expect exhaustiveness, despite
a good deal of small type and bibliography, over which other
readers (for whose sakes technicalities have also been kept
down as much as possible) may lightly skim.
Our central thesis has been, in the first place, to present an
outline of the main processes for the continuance of organic
life with such unity as our present knowledge renders possible ;
and in the second, to point the way towards the interpretation
of these processes in those ultimate biological terms which
physiologists are already reaching as regards the functions of
individual life, — those of the constructive and destructive
changes (anabolism and katabolism) of living matter or proto-
plasm. «
X PREFACE.
But while Books I. and II. are thus the more important,
and such chapters as " Hermaphroditism," ** Parthenogenesis,"
" Alternation of Generations," have only a subordinate and
comparatively technical interest, it will be seen that our theme
raises nearly all the burning questions of biology. Hence, for
instance, a running discussion and criticism of the speculative
views of Professor Weismann, to which their very recent intro-
duction to English readers* has awakened so wide an interest.
At once of less technical difficulty, and in some respects even
wider issues, is the discussion of Mr Darwin's theory of sexual
selection, reopened by the other leading contribution to the
year's biological literature which we owe to Mr Alfred Russel
Wallace.! Besides entering this controversy at the outset of the
volume, we have in the sequel attempted to show that the view
taken of the processes concerned with the maintenance of the
species leads necessarily to a profound alteration of our views
regarding its origin, although the vast problems thus raised
necessarily remain open for fuller separate treatment It is right,
however, to say that the restatement of the theory of organic
evolution, for which we here seek to prepare (that not of indefi-
nite but definite variation, with progress and survival essen-
tially through the subordination of individual struggle and
development to species-maintaining ends), leads us frankly to
face the responsibility of thus popularising a field of natural
knowledge from which there are so many superficial reasons to
shrink, and which knowledge and ignorance so commonly
conspire to veil. For if not only the utmost degeneracy be
manifestly connected with the continuance of organic species,
but also the highest progress and blossoming of life in all its
forms, of man or beast or flower, it becomes the first practical
«- t.
Heredity." Oxford, 1889. t *' Darwinism.'' Lond. 1889.
PREFACE. Xi
application of biological science not only to investigate and
niap out these two paths of organic progress, but to illuminate
them. Hence we have attempted to indicate the application of
the general organic survey, which has been our main theme, to
such questions as those of human population and progress,
although here, more even than elsewhere, our treatment can be
at best only suggestive, not exhaustive. While limits of space
have made it impossible to give the botanical side of our sub-
ject its proportionate share of attention, our illustrations of the
essential facts are sufficient to show the parallelism of the
reproductive processes throughout nature.
It remains to express our thanks to Professor F. Jeffrey
Bell for some valuable suggestions while the work was passing
through the press ; to Mr G. F. Scott-Elliot for assistance in
summarising certain portions of the literature; and to our
engravers, Messrs Harry S. Percy, F. V. M*Combie, and G.
A. Morison, especially to the first-named, who has executed
the great majority of our illustrations with much care and skill.
. PATRICK GEDDES.
J. ARTHUR THOMSON.
t'
K.
\t
CONTENTS.
BOOK I.— MALE AND FEMALE.
CHAPTER I.
PAGE
The Sexes and Sexual Selection - - - - 3-15
§ I. Primary and secondary sexual characters - - 3
§ 2. Illustrations from Darwin .... c
§ 3. Darwin's explanation by sexual selection - - 8
§ 4. Criticisms of Darwin's explanation ... 10
(a.) Wallace ------ 10
(d,) Brooks - - - - - - ix
{c.) St George Mivart - - - - 13
CHAPTER II.
The Sexes, and Criticism of Sexual Selection - 16-33
§ I. Sex-differences - - - - - - 16
§ 2. Differences in general habit. Males more active,
females more passive - • - • - 16
§ 3. Differences in size. Males smaller, females larger.
Pigmies and exceptions • • - • 19
§ 4. Secondary differences in colour, skin, &c. Males
relatively more katabolic, females relatively more
anabolic .--... 22
§ 5. Sexual selection : Its limits as an explanation - - 27
Postulate of extreme aesthetic sensi-
tiveness - - - - 29
Darwin and Wallace combined and
supplemented - • - 31
Sexual selection a minor accelerant,
natural selection a check to ex-
tremes of variation • - 31
XIV CONTENTS.
CHAPTER III.
PAGE
The Determination of Sex (Hypotheses and Obser-
vations) ------- 34-44
§ I. The period at which sex is settled. Floss, Suttou,
Laulani^, &c. ..... 34
§ 2. Over five hundred theories suggested —
Theological.
Metaphysical.
Statistical and hypothetical.
Experimental. (Chap. IV.)
§ 3. Theory of male and female ova is no solution - • 36
§4. Theory of "polyspermy," or multiple fertilisation,
dismissed --..-- 36
§ 5. The theory of age of elements allowed. Thury,
Hensen, &c -.---- 36
§ 6. Theory of parental age of secondary moment. Hofacker
and Sadler - - - - - - 37
§ 7. Theories of *' comparative vigour," &c., require analysis 38
I 8. Theory of Starkweather,— -many factors combined under
"superiority" ..... 39
§ 9. Darwin's position - ... - 40
§ la DUsing's synthetic treatment, and theory of self-regula-
tion of numbers .... - 40
§11. The sexes of twins - - - - - 41
CHAPTER IV.
The Determination of Sex (Constructive Treatment) 45-59
§ I. Nutrition as a factor determining sex. Favourable
nutrition tends to females . - . . 45
(fl.) Yung's tadpoles - - - "45
{6.) Case of bees .... 46
(c. ) Von Siebold's observations • - - 48
(d. ) Case of aphides .... 49
{e, ) Butterflies and moths - . . 50
(/. ) Crustaceans - - . - ... 50
ig.) Rotifers - - • - - 51
(i.) Mammals - - - - - 51
(1.) Human species - - - - 51
(/) Plants ..... 52
§ 2. Temperature as a factor. Favourable conditions tend to
females ...... 53
§ 3. Summary of factors : —
(a.) Nutrition, age, &c., of parents affecting —
{d. ) Condition of sex-cells, followed by —
(c.) Environment of embryo ... 54
§ 4. General conclusion : — Anabolic conditions favour pro-
duction of females, katabolic conditions males • 55
§ 5. Hence corroboration of conclusion of Chap. II., that fe-
males were preponderatingly anabolic, males katabolic 55
§ 6. Notes on Weismann*s theory of heredity - - 55
CONTENTS.
XV
BOOK II.— ANALYSIS OF SEX— ORGANS, TISSUES,
CELLS.
CHAPTER V.
Sexual Organs and Tissues
§ I . Essential sexual organs of animals
§ 2. Associated ducts
§ 3. Origin of yolk -glands, &r.
§ 4. Organs auxiliary to impregnation
§ 5- Egg-laying organs
§ 6. Brood-pouches •
CHAPTER VL
Hermaphroditism ......
§ I. Definition of hermaphroditism ; its varied forms
§ 2. Embryonic hermaphroditism. Ploss, Laulani^, Sutton
§ 3. Casual or abnormal hermaphroditism, from jelly-fish to
mammal ---.-.
§ 4. Partial hermaphroditism, from butterflies to birds
§ 5. Normal adult hermaphroditism, from sponges to toads
§ 6. Degrees of normal hermaphroditism -
§ 7. Self-fertilisation and its preventives
§ 8. Complemental males — cirripedes and Myzostomata
§ 9. Conditions of hermaphroditism; its association with
passivity and parasitism ....
§ la Origin of hermaphroditism ; a primitive or a secondary
condition ---.--
CHAPTER VH.
The Sex-Elements (General and Historical) -
§ I. The ovum theory ....
§ 2. The history of embryology, "evolution" and "epi
genesis" - - - . .
{a.) Harvey's epigenesis and prevision of ovum
theory - - - -
(d.) Malpighi and early observers
(c) Preformation school; "evolution" accord
ing to Haller, Bonnet, and Buffon
ovists and animalculisls -
(t/,) Wolffs demonstration of epigenesis
(^. ) Wolffs successors -
§ 3. The ce'.l-theory ....
§ 4. The protoplasmic movement -
§ 5. Protozoa contrasted with Metazoa ; the making of the
"body"
§ 6. General origin of the sex-cells in sponges
I, „ ,, ,, coelenterates
,, ,, ,, „ other Metazoa
§ 7. Early separation of the sex-cells in a minority of cases
PAGE
63-70
63
66
67
67
68
69
71-86
71
71
72
72
79
82
83
84
87-103
88
88
88
89
89
91
92
92
92
9S
97
97
97
98
XVI CONTENTS.
§ 8. *' Body " versus repioductive cells, and the continuity
of the latter -
(a,) Owen
(6.) Haeckel and Rauber
{c.) Brooks
(</.)Jager
(«.) Gallon
{/,) Nussbaum •
(g.) Weismann -
§ 9. Weismann*s theory of the continuity of the germ -plasma
PAGE
99
99
100
100
100
100
100
ICX)
lOI
CHAPTER VIII.
The Egg-Cell or Ovum . . - - - 104-116
§ I. Structure of ovum —
Cell-substance and protoplasm • - - 104
Nucleus and chromatin - ■ - - 105
§ 2. Growth of ovum —
Transition from amoeboid to encysted phase - 106
§ 3. The yolk-
Its threefold mode of origin - - • 107
Its diffuse, polar, or central disposition - • 108
Resulting influence on segmentation - - 109
§ 4. Composite ova < - - ■ - - no
§ 5* Egg-envelopes —
(a.) From ovum itself - - - • no
(^.) From surrounding cells - - - no
(c) From special glands - - - no
§ 6. Birds' eggs —
Concrete illustration of fieicts and problems • no
§ 7. Chemistry of the ovum —
Its capital of anastates - - • - in
§ 8. Maturation of ovum —
Occurrence, formation, history of polar globules ;
parthenogenetic ova - - - • 112
§ 9. Theories of polar globules —
(tf.) Minot, Balfour, &c. - • - n3
(d.) Giard, Mark, Butschli, &c. - • n4
{c.) Weismann, Hertwig, &c. - - - 114
CHAPTER IX.
The Male-Cell or Spermatozoon -
§ I. General contrast between sperm and ovum—
An index to contrast between male and female
§ 2. History of Discovery—
(a.) Hamm and Leeuwenhoek -
(A.) Animalculists
{c. ) Classed as Entozoa or parasites
(d,) Kolliker*s demonstration of cellular origin
117-125
117
117
117
n7
118
CONTENTS,
§ 3. Structure of spermatozoon —
•• Head," " tail," "middle portion," &c.
§4. Physiology of spermatozoon —
Locomotor energy and persistent vitality
§ 5. Origin of spermatozoa —
Theory of spermatogenesis
§ 6. Further comparison of sperm and ovum —
Processes comparable with formation of polar
globules .....
§ 7. Chemistry of the sperm ....
xvil
PAOB
118
120
121
123
123
CHAPTER X.
Theory of Sex: Its Nature and Origin
§ I. Suggested theories of male and female —
Rolph
Minot
Brooks -
§ 2. Nature of sex — seen in Sex-cells
The cell-cycle
Protoplasmic interpretation
§3. Problem of origin of sex
§ 4. Nature of sex as seen in its origin among plants
§ 5. Nature of sex as seen in its origin among animals
§ 6. General conclusions from forgoing chapters -
126-142
126
127
127
127
127
135
136
139
BOOK III.— PROCESSES OF REPRODUCTION.
CHAPTER XI.
Sexual Reproduction
81.
§2.
§3.
§4-
§5.
§6.
§7.
§8.
Different modes of reproduction
Facts involved in sexual reproduction -
Fertilisation in plants —
From Sprengel to Strasburger -
Fertilisation in higher animals —
From Martin Barry and Bdtschli to Van 'Bene-
den and Boveri ....
Fertilisation in Protozoa . - . -
Origin of fertilisation —
(a.) Plasmodium ....
(^.) Multiple conjugation ...
(f.) Ordinary conjugation
(d,) Union of incipient ly dimorphic cells
(e.) Fertilisation by differentiated sex-cells
Hybridisation in animals and plants -
Tclegony ......
145-168
MS
145
146
ISO
157
IS9
159
160
161
161
162
166
XVlll
CONTENTS.
CHAPTER XII.
Theory of Fertilisation
PAOB
169-182
§ I. Old theories —
{a) Ovists, (5) animalciiHsts, {c) the ** aura
semina/is" - - . -
§ 2. Modern morphological theories —
Nuclei all-important. Herlwig, Strasburger, &c
Cell-substance also important. Nussbaum
Boveri, &c. - - - -
§ 3. Modem physiological theories —
Sachs, De Bary, Marshall Ward, &c. -
Cienkowski and Kolph -
Weismann*s view
Critique and statement of present theory
§ 4. Use of fertilisation to the species —
Rejuvenescence —
Van Beneden and Biitschli •
Oalton and Hensen -
Weismann's critique
The observations of Maupas
A source of variation. Brooks and Weismann
169
171
171
172
173
173
173
175
175
175
176
176
179
CHAPTER XIII.
Degenerate Sexual Reproduction or Parthenogenesis
§ I. History of discovery - - . -
§ 2. Degrees of parthenogenesis —
Artificial, pathological, occasional, partial, sea
sonal, juvenile, total
§ 3. Occurrence in animals —
Rotifers, crustaceans, insects
§ 4. Occurrence in plants —
Phanerogams and fungi -
§ 5. The offspring of parthenogenesis
§ 6, Effects on the species - - . -
§ 7. Peculiarities of parthenogenctic ova —
Weismann's discovery -
§ 8. Theory of parthenogenesis —
(a.) Balfour ....
{d.) Minot - . . -
[c) Rolph ....
(J.) Strasburger - - - -
{e.) Weismann - - - -
The theory proposed
§ 9* Origin of parthenogenesis
183-199
183
184
188
190
191
192
193
194
194
195
195
195
196
197
CONTENTS.
XIX
CHAPTER XIV.
Asexual Reproduction
§ I. Artificial division
§ 2. Regeneration - - - -
$ 3. Degrees of asexual reproduction
§ 4. Asexual reproduction in plants and animals
PAGB
200-212
200
201
203
203
CHAPTER XV.
Alternation of Generations
§1.
§2.
§3.
§4-
§5-
§6.
S7.
§8.
§9.
§ 10.
§ II.
§ 12.
§13-
History of discovery - - - -
Rhythm between sexual and asexual reproduction
Alternation between sexual and degenerate sexual repro
duction . . - . -
Combination of both these alternations
Alternation of juvenile parthenogenetic reproduction
with the adult sexual process
Alternation of parthenogenesis and ordinary sexual
reproduction . . . -
Alternation of different sexual generations
Occurrence of these alternations in animals
Beard's hypothesis of alternation of generations in
vertebrates -----
Occurrence of alternations in plants
The problem of heredity in alternating generations
Hints as to the rationale of alternation
Origin of alternation of generations
213-229
213
214
216
219
220
220
221
221
223
223
224
225
226
BOOK IV.— THEORY OF REPRODUCTION.
CHAPTER XVI.
Growth and Rlproduciion
§1.
§2.
§3.
§4-
§5-
§6.
§7.
Facts of growth
Spencer's theory of growth
Cell-division ....
Protoplasmic restatement
Antithesis between growth and reproduction
The contrast in the individual —
{a,) In distribution of organs
(^.} In the periods of life
The contrast between asexual and sexual reproduction
233-245
233
234
235
237
238
239
240
241
CHAPTER XVII.
Theory of Reproduction — coniinued
§ I. The essential fact in reproduction
§ 2. The beginnings of reproduction
§ 3. Cell-division
246-252
246
246
247
XX
CONTENTS.
§ 4. Gradations from asexual severance to liberation of sex
cells -..--.
§ 5. The close connection between reproduction and death
§ 6. Reproduction as influenced by the environment
§ 7. General conclusion ....
PACB
247
248
249
250
CHAPTER XVIII.
Special Physiology of Sex and Reproduction
§ I. The continuity of the germ-plasm
§ 2. Sexual maturation
§ 3. Menstruation
§ 4. Sexual union
§ 5. Parturition
§ 6. Early nutrition
§ 7. I^actation
§ 8. Other secretions
§ 9. Incubation
§ ID. Nemesis of reproduction
§ 1 1. Organic immortality
253-282
253
25s
259
261
263
265
266
267
268
272
275
CHAPTER XIX.
Psychological and Ethical Aspkcts
§ I. Common ground between animals and men
§ 2. The love of mates ....
§ 3. Sexual attraction ....
§ 4. Intellectual and emotional differences between the
sexes ..-.--
§ 5. Ix)ve for offspring . . . -
§ 6. Egoism and altruism ....
283-298
283
283
285
286
291
295
CHAPTER XX.
Laws of Multiplication
§ I. Rate of reproduction and rate of increase
§ 2. History of discussion -
§ 3. Spencer's analysis ; individuation and genesis
§ 4. Spencer's application to man •
I 5. General statement of the population question
§6. Sterility ....
299-3 IS
299
300
300
304
305
314
CHAPTER XXI.
The Reproductive Factor in Evolution - - 316-333
§1. General history of evolution . - . . ■ 316
I 2. The reproductive factor so far as hitherto recognised - 323
§ 3. Suggested lines of further construction • - 326
BOOK I.
» ♦ t
THE SEXES AND SEXUAL SELECTION.
THE EVOLUTION OF SEX.
->ts-
CHAPTER I.
The Sexes and Sexual Selection.
THAT all higher animals are represented by distinct male
and female forms, is one of the most patent facts of
observation, striking enough in many a beast and bird to catch
any eye, and familiarly expressed in not a few popular names
which contrast the two sexes. In lower animals, the contrast,
and indeed the separateness, of the sexes often disappears; yet
even naturalists have sometimes mistaken for different species,
what were afterwards recognised to be but the male and female
of a single form.
§ I. Primary and Secondary Characters, — When we pass
from this commonplace of observation and experience to inquire
more precisely into the differences between the sexes, we speedily
recognise that these are of very different degrees. In some cases
no marked differences whatever are recognisable ; thus a male
star-fish or sea-urchin looks exactly like the female, and a care-
ful examination of the essential reproductive organs is requisite
to determine whether these respectively produce male elements
or eggs. In other cases, for instance in most reptiles, no
external differences are at all striking, but the aspect of the
internal organs, both essential and auxiliary to reproduction, at
once settles the question. In a great number of cases, again,
the sexes resemble one another closely, but each has certain
minor structural features at once decisive as to its respective
maleness or femaleness. Thus in the males there are
frequently prominent organs used in sexual union, while the
peculiar functions of the females are indicated in the special
egg-laying or young-feeding organs. All such characters,
THE EVOLUTION OF SEX.
directly associated with the essential functions of the sexes,
are included under the title oi primary sexual characters.
Of less real importance, though often much more striking,
are the numerous distinctions in size, colour, skin, skeleton, and
the like, which often signaUse either sex. These are termed
secondary sexual characters ; for though they will be shoivn in
THE SEXES AND SEXUAL SELECTION. S
some cases at least to be truly part and parcel with the male or
female constitution, they are only of secondary importance in
the reproductive process. The beard of man and the mane
of the lion, the antlers of stags and the tusks of elephants, the
gorgeous plumage of the peacock or of the bird of paradise,
are familiar examples of secondary sexual characters in males.
Nor are the females lacking in special characteristics, which
serve as indices of their true nature. Large size is one of the
commonest of these ; while in some few^ cases the excellencies
of colour, and other adornments, are possessed by the females
rather than by their mates.
The whole subject of secondary sexual characters has found
its most extensive treatment in Darvv-in's "Descent of Man,"
and to that work, therefore, the more so as its limits exceed
those of the present volume, the reader must be assumed to
make reference. All that can be here attempted is an illustra-
tion, by representative cases, of the main differences between
the sexes ; from which we shall pass to Darwin's interpretation,
and, after a fresh survey, to a re-statement from another point
of view.
§ 2. Illustrations from Darwin. — Among invertebrates,
prominent secondary sexual characters are rarely exhibited
outside the great division of jointed-footed animals or arthro-
pods. There, however, among crustaceans and spiders, but
especially among insects, beautiful illustrations abound. Thus
the great claws of crabs are frequently much larger in the
males; and male spiders often differ from their fiercely coy
mates, in smaller size, brighter colours, and sometimes in the
Winged Male and Wingless Female of a Moth
iOrgyia aHttgita%-»-Ftoai Leunis.
power of producing rasping sounds. Among insects, the males
are frequently distinguished by brighter colours attractively dis-
played, by weapons utilised in disposing of their rivals, and
by the exclusive possession of the power of noisy love-calling.
Thus, as the Greek observed, the cicadas " live happy, having
voiceless wives." Not a few male butterflies are pre-eminently
THE EVOLUTION OF SEX.
more brilliant than the females ; and many male beetles fight
savagely for the possession of their mates.
Passing to backboned animals, we find that among fishes
the males are frequently distinguished by bright colours and
ornamental appendages, as well as by structural adaptations for
combat. Thus the "gemmeous dragonet " {Callionymus lyra) is
flushed with gorgeous colour, in great contrast to the " sordid "
female, and is further adorned by a graceful elongation of the
dorsal fin. In many cases, as in the sea-scorpion {Coitus
scorpim\ or in the stickleback {Gasterosteus), it is only at the
reproductive period that the males are thus transformed,
literally putting on a wedding-garment. Every one knows, on
the other hand, the hooked lower jaw of the male salmon,
which comes to be of use in the furious charges between rivals;
and this is but one illustration of many structures utilised in
the battle for mates. In regard to amphibians, it is enough to
recall the notched crests and lurid colouring of our male newts,
and the indefatigable serenading powers of male frogs and
toads, to which the females are but weakly responsive. Among
reptiles, differences of this sort are comparatively rare, but male
snakes have often more strongly-pronounced tints, and the
scent-glands become more active during the breeding season.
In this, as in many other cases, love has its noisy prayer re-
placed by the silent appeal of fragrant incense. Among lizards,
the males are often more brightly decorated, the splendour of
their colours being frequently exaggerated at pairing time.
They may be further distinguished by crests and wattle-like
pouches ; while horns, probably used in fighting, are borne by
some male chamaeleons.
It is among birds, however, that the organic apparatus of
courtship is most elaborate. The males very generally excel in
brighter colours and ornaments. Beautiful plumes, elongated
feathery tresses, brightly-coloured combs and wattles, top-knots
and curious markings, occur with marvellous richness of variety.
These are frequently displayed by their possessors before the
eyes of their desired mates, with what seem to us like emotions
of love and vanity. Or it may be to the subtler charms of music
that the wooers mainly trust. During the breeding season, the
males are jealously excited and pugnacious, while some have
special weapons for dealing directly with their rivals. The differ-
ences between the magnificent male birds of paradise and their
soberly- coloured mates, between the peacock with his hundred
THE SEXES AND SEXUAL SELECTION. J
eyes and ihe plain peahen, between the musical powers of
male and female songsters, are very familiar facts. Or again,
the combs and " gills " of cocks, the " watiles " of turkey-cocks,
the immense top-knot of the male umbrella-bird (Ctphalopterus
ernalus), the throat-pouch of the bustard, — illustrate another
Blidicock and Crty Hen (Milt and Fenule).
series of secondary sexual characters. The spurs ol cocks and
allied birds are the most familiar illustrations of weapons used
by the males in fighting with rivals. As in other animals, it is
important to notice that male birds often acquire their special
secondary characters, such as colour, markings, and special
forms of feathers, only as they approach sexual maturity, and
sometimes retain them in all their glory only during the
breeding season.
Among mammals, which stand in so many ways in marked
contrast to birds, the law of battle much more than the power
of charming decides the problem of courtship. Thus most
of the striking secondary characters of male mammals are
weapons. Yet there are crests and tufts of hair, and other
acknowledgments of the beauty test, while the incense of
odoriferous glands is a very frequent means of sexual attrac-
6
THE EVOLUTION OF SEX.
tion. The colours too of the males are often more sharply
contrasted, and there are minor differences, in voice and the
like, which cannot be ignored. Of weapons, the larger canine
teeth of many male animals, such as boars ; the special tusks
of, for instance, the elephant and nar>vhal ; the antlers of stags,
The development of antlers in the successive years of a
stag's life, or in the general history of stages. — From
Carus Sterne.
all but exclusively restricted to the combative sex; the
horns of antelopes, goats, etc., — which are usually much
stronger in the males, — are well-known illustrations. The
manes of male lions, bisons, and baboons; the beards of
certain goats ; the crests along the backs of some antelopes ;
the dewlaps of bulls, — illustrate another set of secondary
characters. The odoriferous glands of many mammals are more
developed in the males, and become specially functional during
the breeding season. This is well illustrated in the case of
goats, deer, shrew-mice, elephants. The differences in colour
are slight compared with those seen between the sexes in birds,
but in not a few orders the distinction is marked enough, males
being, in the great majority of cases, the more strongly and
brilliantly coloured. Among monkeys the difference in colour
in the bare regions, and the subtler decorations in the arrange-
ment of the hair on the face, are often very conspicuous.
§ 3. DanvirCs Explanation — Sexual Selection, — Darwin
started from the occurrence of such variations, in structure and
habit, as might be useful either for attraction between the sexes
or in the direct contests of rival males. The possessors of
these variations succeeded better than their neighbours in the
art of courtship; the factors which constituted success were
transmitted to the offspring ; and, gradually, the variations were
THE SEXES AND SEXUAL SELECTION. 9
established and enhanced as secondary sexual characters of the
species. The process by which the possessors of the fortunate
excellencies of beauty and strength outbid or overcome their
less endowed competitors, he termed " sexual selection." It is
only fair, however, to state Mr Darwin's case by direct
quotation.
Sexual selection " depends on the advantage which certain
individuals have over others of the same sex and species solely
in respect of reproduction." ... In cases where " the males
have acquired their present structure, not from being better
fitted to survive in the struggle for existence, but from having
gained an advantage over other males, and from having trans-
mitted this advantage to their male offspring alone, sexual
selection must have come into action." ... "A slight degree
of variability, leading to some advantage, however slight, in
reiterated deadly contests, would suffice for the work of sexual
selection." ... So too, on the other hand, the females " have,
by a long selection of the more attractive males, added to their
beauty or other attractive qualities." ..." If any man can in
a short time give elegant carriage and beauty to his bantams,
according to his standard of beauty, I can see no reason to
doubt that female birds, by selecting during thousands of
generations the most melodious or beautiful males, according
to their standard of beauty, might produce a marked effect."
..." To sum up on the means through which, as far as we
can judge, sexual selection has led to the development of
secondary sexual characters. It has been shown that the
largest number of vigorous offspring will be reared from the
pairing of the strongest and best-armed males, victorious in
contests over other males, with the most vigorous and best-
nourished females, which are the first to breed in the spring.
If such females select the more attractive, and at the same
time vigorous males, they will rear a larger number of offspring
than the retarded females, which must pair with the less
vigorous and less attractive males. So it will be if the more
vigorous males select the more attractive, and at the same time
healthy and vigorous females; and this will especially hold
good if the male defends the female, and aids in providing
food for the young. The advantage thus gained by the more
vigorous pairs in rearing a larger number of offsi)ring, has
apparently sufficed to render sexual selection efficient."
Another sentence from Darwin's first statement of his position
lO THE EVOLUTION OF SEX.
must, however, be added. " I would not wish," he says in the '
"Origin of Species," "to attribute all such sexual differences ,
to this agency; for we see peculiarities arising and becoming
attached to the male sex in our domestic animals, which we
cannot believe to be either useful to the males in battle or
attractive to the females." ^
§ 4. Criticisms of Darwin^ s Explanation, — The above *
explanation may be summed up in a single sentence, — a con-
genital variation, advantageous to its possessor (usually a male)
in courtship and reproduction, becomes established and per-
fected by the success it entails. Sexual selection is thus only
a special case of the more general process of natural selection,
with this difference, that the female for the most part takes the
place of the general environment in the picking and choosing
which is believed to work out the perfection of the species.
The more serious objections which have been hitherto
urged against this hypothesis, apart altogether from criticism
of special cases, are the following : — (a) Alfred Russel Wallace 1
and others would explain the facts on the more general theory l
of natural selection, allowing comparatively little import to the t
alleged sexual selection exercised by the female. ijH) Some,
who allow great importance to both natural and sexual selec-
tion, are not satisfied with leaving variation a mere postulate.
The position occupied by Brooks will be sketched below,
(r) Different from either of the above is the position occupied
by St George Mivart and others, who attach comparatively
little importance to either natural or sexual selection, but seek
in terms of definite variation, constitutional tendencies, laws of
growth, — for the idea is very variously expressed, — to find the
primary and fundamental interpretation of sexual dimorphism.
(a) Wallaces Objection, — It is convenient to begin with
Wallace's early criticism, which precedes that of Brooks in
chronological order. This is the more helpful in clearing the
ground, since the two theories of Wallace and Darwin are
strikingly and, at first sight, irreconcilably opposed. Accord-
ing to Darwin, the gayness of male birds is due to selection on
the part of the females ; according to Wallace, the plainness of \
female birds is due to natural selection, which has eliminated «
those which persisted to the death in being gay. He points
out that conspicuousness during incubation would be dangerous
and fatal ; the more conspicuous have, he thinks, been picked
off their nests by hawks, foxes, and the like, and hence only
THE SEXES AND SEXUAL SELECTION. II
the sober-coloured females now remain. Darwin starts from
inconspicuous forms, and derives gorgeous males by sexual
selection; Wallace starts from conspicuous forms, and derives
the sober females by natural selection ; the former trusts to the
preservation of beauty, the latter to its extinction. In 1773,
the Hon. Daines Barrington, a naturalist still remembered as
the correspondent of Gilbert White, suggested that singing-
birds were small, and hen-birds mute for safety's sake. This
suggestion Wallace has repeated and elaborated in reference
especially to birds and insects. The female butterfly, exposed
to danger during egg-laying, is frequently dull and inconspicuous
compared with her mate. The original brightness has been
forfeited by the sex as a ransom for life. Female birds in open
nests are similarly, in many cases, coloured like their surround-
ings; while in those birds where the nests are domed or
covered, the plumage is gay in both sexes.
But in his book on " Darwinism " Wallace goes much
further in his destructive criticism of Darwin's sexual selection.
The phenomena of male ornament are discussed, and summed
up as being " due to the general laws of growth and develop-
ment," and such that it is " unnecessary to call to our aid so
hypothetical a cause as the cumulative action of female prefer-
ence." Or again, "if ornament is the natural product and
direct outcome of superabundant health and vigour, then no
other mode of selection is needed to account for the presence
of such ornament" This mode of criticism, however, belongs
to our third category.
(d) Brooks has called attention to the sexual differences in
lizards, where the females do not incubate ; or in fishes, where
the females are even less exposed to danger than the males ;
or in domesticated birds, where, though all danger is removed,
the males are still the more conspicuous and diversified sex.
" The fact too that many structures, which are not at all con-
spicuous, are confined, like gay plumage, to male birds, also
indicates the existence of an explanation more fundamental
than the one proposed by Wallace, and the latter explanation
gives no reason why the females of allied species should often
be exactly alike when the males are very different" To the
explanation which Brooks proposes we must therefore pass.
According to Darwin, Brooks says, the greater modification
of the males is due to their struggling with rivals, and to their
selection by the females, but " I do not believe that this goes
12 ^HE EVOLUTION OF SEX.
to the root of the matter." The study of domesticated pigeons,
for instance, shows that " something within the animal deter-
mines that the male should lead and the female follow in
the evolution of new breeds." The same is true in other
domesticated animals, where, from the nature of the circum-
stances, it is inadmissible to explain this with Darwin, by
supposing that the male is more exposed than the female to
the action of selection, whether natural or sexual. Darwin
concludes, indeed, that the male is more variable than the
female, but he gives no satisfactory reason why female varia-
tions should be less apt than male variations to become
hereditary, or, in other words, why the right of entail is so
much restricted to the male sex. Darwin merely attributes
this to the greater eagerness of the males, which " in almost all
animals have stronger passions than the females." The theory
which Brooks maintains, is bound up with an hypothesis of
heredity differing considerably from that held by Darwin. He
supposes that the cells of the body give off gem mules, chiefly
during change of function or of environment, and that "the
male reproductive cell has gradually acquired, as its especial
and distinctive function, a peculiar power to gather and store
up these gemmules." The female reproductive cells keep up
the general constancy of the species, the male cells transmit
variations. "A division of physiological labour has arisen
during the evolution of life, and the functions of the repro-
ductive elements have become specialised in different direc-
tions." "The male cell became adapted for storing up
gemmules" (the results of variations in the body), "and at the
same time gradually lost its unnecessary and useless power to
transmit hereditary characteristics." "We thus look to the
cells of the male body for the origin of most of the variations
through which the species has attained to its present organisa-
tion." The males are the more variable, but more than that,
their variations are much more likely to be transmitted. "We
are thus able to understand the great difference in the males
of allied species, the difference between the adult male and the
female or young, and the great diversity and variability of
secondary male characters; and we should expect to find, what
actually is the case, that among the higher animals, when the
sexes have long been separated, the males are more variable
than the females." The contrast between Darwin and Brooks
may now be summed up again in a sentence. Darwin says,
THE SEXES AND SEXUAL SELECTION. 1 3
the males are more diversified and richer in secondary sexual
characters, chiefly because of the sexual selection exercised
alike in courtship and in battle. Brooks admits sexual
selection, but believes that the males are naturally or con-
stitutionally more variable than the females, and that it is the
peculiar function of the male elements to transmit variations,
as opposed to the constant tradition of structure kept up by
the egg-cells or ova. In other words, the females may choose,
yet the males lead ; nay more, they must lead, for male varia-
tions have by hypothesis most likelihood of being transmitted.
Full consideration of this hypothesis would involve much
discussion of the problems of inheritance, but the general con-
clusion of the naturally greater variability of the males, will be
stated in a different light towards the close of the following
chapter. It will there be shown that the "something within
the animal," which determines the preponderance of male
variability, may be stated in simpler terms than are involved in
Brooks's theory. Moreover, the greater variability of the males,
which seems quite plain when we contrast, for instance, ruffs
and reeves, requires to be proved for each case. Karl Pearson
has disproved it for man.
Somewhat similar to Wallace's later position is that (r)
occupied by St George Mivart, who also looks for some deep
constitutional reason for sexual dimorphism. The entire
theory of sexual selection appears to him an unverified hypo-
thesis, only acquiring plausibility when supported by numerous
subsidiary suppositions. He submits a number of detailed
criticisms; but his chief contention is, that the beauty of
males, and other secondary sexual characters, are not the
indirect results of a long process of external selection, but the
direct expressions of the internal differences of a progressively
varying internal constitution, i>., of tendencies inherent in the
individual.
Mivart's position and the vague suggestions of Mantegazza
and others are of importance as indications of progress towards
a fundamental re statement. As we have seen, an obvious
objection to the theory of sexual selection is that, while it may
in part account for the persistence and progress of secondary
characters after they attained a certain degree of development,
it does not account for their preservation when weak or incon-
spicuous. In short, the theory may account for the perfecting,
but not for the origin of the characters. It may be enough to
14 THE EVOLUTION OF SEX.
account for the length and the trimmings of the living garment,
but what we wish to know is the secret of the loom. Darwin's
account of the evolution of the eyes on the feathers of the Argus
pheasant is indeed ingenious and interesting; but, whatever its
probability, it is more important to ask what the predominant
brightness of males means as a general fact in physiology. It
is of interest, then, to notice the hints thrown out by Mante-
gazza, Wallace, and others, directly associating decorativeness
with superfluous reproductive material, and the putting on of
wedding-robes with the general excitement of the sexually
mature organism. From this record of the discussion, it is
time however to turn to a more constructive mode of treat-
ment
In passing, however, it should be noted that the fact of
preferential or selective mating can be proved not only by
observation, as the Peckhams have done in the case of spiders,
but more conclusively by statistical enquiry, by investigating
" whether the type and variability of the mated and unmated
members of one or other sex are the same " (see Karl Pearson's
"Grammar of Science," 2nd ed., 1900, p. 425). Apart from
the particular problem of secondary sexual characters, it is of
the utmost importance whether the mating is selective or in-
discriminate. For if natural selection is at work its effect will
be checked by indiscriminate mating, and aided by preferential
or, to use the wider term, selective mating.
THE SEXES AND SEXUAL SELECTION. 1 5
SUMMARY.
I, 2. The existence of male and female animals is a commonplace of
observation. They differ in primary and in secondary sexual characters,
of which illustrations are given, chiefly from Darwin.
3. Darwin's theory of sexual selection seeks to account for the preserva-
tion and perfection of variations, advantageous in courtship or in battles
with rivals.
4. Wallace maintains that the females have been protectively retarded
by natural selection, and at a later date suggests that the greater decora*
tiveness, etc., of the males is an expression of superabundant vigour.
Brooks believes that the males are naturally the more variable, and pre-
dominate in power of transmitting variations. Mivart demands a deeper
analysis than is aflbrded by either sexual or natural selection. This physio-
logical rationale is hinted at.
LITERATURE.
Brooks, W. K. — The Law of Heredity: A Study of the Cause of Varia-
tion and the Origin of Living Organisms. Baltimore, 1883.
Cunningham, J. T. — Sexual Dimorphism. London, 190a
Lb Dantec, F. — La Sexuality. Paris, 1899, 98 pp.
Dakwin, C — On the Origin of Species by Means of Natural Selection;
or, The Preservation of Favoured Races in the Struggle for Life.
London, 1859.
• The Descent of Man, and Selection in Relation to Sex. London,
1871.
Delage, Y. — La structure du protoplasma, et les theories de I'h^r^dit^ et
les grands probl^mes de la biologie g^n^rale. Paris, 1895.
Ellis, Havelogk. — Man and Woman (Contemporary Science Series).
Geddes, P., and Thomson, J. A. — L'^volution du sexe. Quelques
probabilit^s biologiques. Revue de Morale Sociale, I. 1899, pp. 95- 1 1 1.
MiVART, St George.— Lessons from Nature. London, 1876. Cf. also
The Genesis of Species. London, 1871.
Morgan, C. Lloyd. — Animal Life and Intelligence. London, 1890-91.
Pearson, K.— Grammar of Science (revised edition). London, 1900.
Wallace, A. R. — Contributions to the Theory of Natural Selection.
London, 1871.
Darwinism : An Exposition of the Theory of Natural Selection,
with Some of its Applications. London, 1889.
CHAPTER II.
The Sexes, and Criticism of Sexual Selection.
1 1. Sex-Differenets. — To gain a firmer and broader Tounda-
tion on which to base a theory of the differences between the
sexes, it is necessary to take another review of the facts of the
case. Instead of considering the differences as they are ex-
pressed in the successi\-e classes of animals, it will be more
convenient lo arrange Ihcm for themselves, according as they
affect habit, size, length of life, and the hke. The review must
again be merely repre sen ta live, without any attempt at com-
pleteness.
§ 2. General Habit — Let us begin with an extreme yet well-
known case. The female cochineal insect, laden with carmine,
which some have interpreted as a reserve-product, spends
THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 1 7
much of its life like a mere quiescent gall on the cactus plant.
The male, on the other hand, in his adult slate is agile,
restless, and short-lived. Now this is no mere curiosity of the
entomologist, but in reality a vivid emblem of what is an aver-
age truth throughout the world of animals — the preponderating
passivity of the females, the predominant activity of the males.
'l"hese coccus insects are the martyrs of their respective sexes.
Take another illustration, again somewhat extreme. There is
a troublesome threadworm {Heterodera schachtii) infesting the
turnip plant, which parallels in more ways than one the contrast
of the coccus insects. The adult male is agile, and like many
another threadworm ; the adult female, however, is quiescent,
and bloated like a drawn-out lemon. It may be asked, how-
ever, is not this merely the natural nemesis
of parasitism? The life-history answers
this objection. The two sexes are at first
alike, — agile, and resembling most thread-
worms ; they become parasitic, and lose
both activity and nematode form ; but the
interesting fact is further, that the male
recovers himself, while the female remains a
victim. In other insect and worm types
the same story, in less accented characters,
may be distinctly read. In many crusta-
ceans, again, the females only are parasitic;
and while this is in part explained by their
habit of seeking shelter for egg-laying pur-
poses, it also expresses the constitutional
bias of the sex. The insect order of bee
parasites (Strepsiptera) is remarkablefor the
completely passive and even larval character
of the blind parasitic females, while the
adult males are free, winged, and short-lived.
Throughout the class of insects there are Femde cifnJnaanHim, b
numerous illustrations of the excellence mth'"p"pny'™k'(B)
of the males over the females, alike in niach^juM ^o™ i^
muscular power and sensory acuteness. "^^"(,1) of the f«^'
The diverse series of efforts by which the — F'om ciau.,
males of so many different animals, from cicadas to birds,
sustain the love-chorus, aifords another set of illustrations of
pre-eminent masculine activity.
Without multiplying instances, a review of the animal
18 THE EVOLUTION OF SEX.
kingdom, or a perusal of Darwin's pages, will amply confirm the
conclusion that on an average the females incline to passivity,
the males to activity. In higher animals, it is true that the
contrast shows rather in many little ways than in any one
striking difference of habit, but even in the human species the
contrast is recognised. Every one will admit that strenuous
spasmodic bursts of activity characterise men, especially in
youth, and among the less civilised races ; while patient con-
tinuance, with less violent expenditure of energy, is as generally
associated with the work of w
Both WIM of a Flea-lhe Jigger or Chigi»(5an:,l*«'^''^™''""«); Ihe
hmale much iwolltn with eggi.— Fram Leuckan.
For completeness of argument, two other facts, which will
afterwards claim full discussion, may here be simply mentioned.
(a) At the very threshold of sex-difference, we find that a little
active cell or spore, unable lo develop of itself, unites in
fatigue with a larger more quiescent individual. Here, at the
very first, is the contrast between male and female, {i) The
same antithesis is seen, when we contrast, as we shall afterwards
THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 1 9
do in detail, the actively motile, minute, male element of most
animals and many plants, with the larger passively quiescent
female-cell or ovum.
It is possible that the reader may urge as a difficulty against
the above contrast the exceedingly familiar case of the male
bees or " drones." It must be frankly allowed that exceptions
do indeed occur, though usually in conditions which afford a
key to the abnormality. Thus it will be allowed that the
"drones" are in a peculiar position as male members of a very
complex society, in which what is practically a third sex is
represented by the great body of "workers." They are no
more fair examples of the natural average of males, than the
hard-driven wives of the lazy Kaffir are of the normal functions
of women. Nor is the exception even here a real one, for the
drone, although passive as compared with the unsexed workers,
is active when compared with the extraordinarily passive queen.
To the above contrast of general habit, two other items
may be added, on which accurate observation is still unfortun-
ately very restricted. In some cases the body temperature,
which is an index to the pitch of the life, is distinctly lower in the
females, as has been noted in cases so widely separate as the
human species, insects, and plants. In many cases, further-
more, the longevity of the females is much greater. Such a
fact as that women pay lower insurance premiums than do
men, is often popularly accounted for by their greater immunity
from accident ; but the greater normal longevity on which the
actuary calculates, has, as we begin to see, a far deeper and
constitutional explanation.
§ 3. Size, — Among the higher animals, there are curious
alternations in the preponderance of one sex over another in
size. Thus among mammals and birds the males are in most
cases the larger ; the same is true of lizards ; but in snakes the
females preponderate. In fishes, the males are on an average
smaller, sometimes very markedly so, even to the extent of not
being half as large as their mates. Below the line, among
backboneless animals, there is much greater constancy of
predominance in favour of the females. Thus among insects,
the more active males are generally smaller, and often very
markedly ; of spiders the same is true, and the males being
often very diminutive are forced to task their agility to the
utmost in making advances to their unamiable mates. So
again, crustacean males are often smaller than the females ; and
ao THE EVOLUTION OF SEX.
in many parasitic species, what have been well called "pigmy"
males illustrate the contrast in an almost ludicrous degree.
Two cases from aberrant worm types ejiliibit very vividly this same anti-
thesis of size. Among the common rolifers, ihe males are almost always
very different from the females, and much smaller. Sometimes they seem
to have dwindled out ofexlslencealtogelher, for only the females aie known
\PkilBdina, Selijer, CalUdina, Adineld), In Palyartkra platyflera the
male Is "hardly to be distinguished from a Vffrlfcel/a which has become
detached from its sialic." In ffydatina uiita (see fig.) the male is about
a third the size of ihc female, has no alimentary canal, and has only two
or three days of adult life. In the great majority the males are " little
more than perambulating; bags of spermatozoa, though in a few cases, like
JihinBft vilrta, degeneration seems hardly to have liegun (Rousselel,
t897). Even when present, Ihey are not indispensable, foi partheno'
genesis is very general.
Selilivi -iim of X male ini ftmalc RotifH (ffj^alma itnfn).
In a remarkable marine worm, Bontllia, the male remains
like a remote ancestor of the female. It lives parasitically on
or within the latter, and is microscopic in size, measuring in fact
only atwut one hundredth part of the length of its host and mate.
Somewhat similar to the case of Bonellia is that of a vivi-
parous coccus insect {Lecanium hesperidum), where the males
are very degenerate, small, blind, and wingless. In spite of
this condition, perhaps indeed because of it, they are very
THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 21
male, for even the larvae, while still within the mother, have
been shown to contain fully-developed spermatozoa. In a
little ** bear animalcule '* or Tardigrade, Macrobiotus macronyx^
the males are about half the size of the females and decidedly
more active. A particularly interesting case of sexual di-
morphism in molluscs has been described by Professor
E. G. Conklin ("Proc. Acad. Nat. Sci. Philadelphia," 1898,
pp. 435-444, 3 pis.). In Crepidula plana^ which occurs in
Figure of the female Boneliia (from Atlas of Naples Aqtiariiun),
with its parasitic pigmy mole enlarged.
shells tenanted by hermit-crabs {Eupagurus bernhardus\
the female is about fifteen times larger than the male, and
the latter retains throughout life the power of locomotion
possessed by the females in their young stages only. The
females occurring along with the little hermit-crab, Eupa-
gurus iongicarpusy are always dwarfed, having a body volume
only one-thirteenth of the more typical form. The cells of the
dwarfs are of the normal size, but there are fewer of them, and
22 THE EVOLUTION OF SEX.
the eggs are also much less numerous. But this dwarfing is
a mere " modification," and not inherited; it probably depends
upon pressure or upon differences of nutrition and oxygena-
tion. That is to say, the dwarfs are not a race, but are
continually recruited from the young of the giants. It is
interesting, therefore, to note that environmental influences
may modify the female till in size it resembles the normally
dwarfish male; and that the small size of the male implies, as
in the case of the dwarfs, not smaller cells, but a smaller
number of cells. In both dwarfs and males, the process of
cell-division has been inhibited.
Dr T. W. Fulton, Naturalist to the Scottish Fishery Board,
has made valuable statistical observations on the size and
numerical proportions of male and female fishes, (i) The
females are usually considerably more numerous than the
males, and never less numerous except in the angler and the
cat-fish. The proportion of females to males among flat-
fishes ranges from about i : i in the flounder, to about 12:1
in the long rough dab. Among "round" fishes the same
proportion varies from about 3 : 2 in the cod, to 9 : 2 in the
common gurnard. (2) The female is longer and larger among
all the flat-fishes, sometimes by as much as 30 per cent In
cod, haddock, angler, and cat-fish, the males are larger, while
in the whiting the females are slightly larger, and in the
common gurnard decidedly so.
One must not indeed base an argument on extreme cases,
but there is no doubt that up to the level of amphidians at
least the females are generally the larger.
Apparent exceptions occur, it is true, among the higher
animals. In birds and mammals the males are usually rather
larger than the females. This difference consists especially in
larger bones and muscles. The apparent exception is in part
the natural result of the increased stress of external activities
which are thrown upon the shoulders of the males when their
mates are incapacitated by incubation or pregnancy. Further-
more, we must recognise the strengthening influence of the
combats between males, and the effect produced on the accu-
mulative constitution of the females by the increased maternal
sacrifice characteristic of the highest animals.
§ 4. Oihtr Characters. — While it is easy to point to the
general physiological import of large size and the reverse,
physiology is not yet far enough advanced to afford firm foot-
THE SEXES, ANt) CftltlCISM 6F SEXtJAL SELECTldN. 2J
hold in dealing with the details of secondary sexual characters.
It is only possible to point out the path which will eventually
lead us to their complete rationale. The point of view is
simple enough. The agility of males is not merely an adapta-
tion to enable that sex to exercise its functions with relation to
the other, but is a natural characteristic of the constitutional
activity of maleness; and the small size of many male fishes is
not an advantage at all, but simply again the result of the
contrast between the more vegetative growth of the female
and the costly activity of the male. So, brilliancy of colour,
exuberance of hair and feathers, activity of scent-glands, and
even the development of weapons, cannot be satisfactorily
explained by sexual selection alone, for this is merely a
secondary factor. In origin and continued development they
are outcrops of a male as opposed to a female constitution.
To sum up the position in a paradox, all secondary sexual
characters are at bottom primary, and are expressions of the
same general habit of body (or to use the medical term,
diathesis)^ as that which results in the production of male
elements in the one case, or female elements in the other.
This theory of the origin and primary meaning of those
variations which culminate in marked sexual dimorphism is
obviously similar to that adopted by Wallace in his book on
" Darwinism."
Three well-known facts must be recalled to the reader's
mind at this point; and firstly, that in a great number of cases
the secondary sexual characters make their appearance step by
step with sexual maturity itself. When the animal — be it a
bird or insect — becomes emphatically masculine, then it is that
these minor outcrops are exhibited. Thus the male bird of
paradise, eventually so resplendent, is usually in its youth
comparatively dull and female-like in its colouring and
plumage. Very often, too, whether in the wedding-robe of
male fishes or in the scent-glands of mammals, the character
rises and wanes in the same rhythm as that of the reproductive
periods. It is impossible not to regard at least many of the
secondary sexual characters as part and parcel of the sexual
diathesis, — as expressions for the most part of exuberant
maleness.
Secondly, when the reproductive organs are removed by
castration, the secondary sexual characters are often much
modified Thus, as Darwin notes, stags never renew their
i4 THE EVOLUTION O^ SE^.
antlers after castration, though normally, of course, they renew
them each breeding season. The reindeer, where the horns
occur on the females as well, is an interesting exception to the
rulei for after castration the male still renews the growth. This
however merely indicates that the originally sexual characters
have become organised into the general life of the body. In
sheep, antelopes, oxen, &c., castration modifies or reduces the
horns; and the same is true of odoriferous glands. The
parasitic crustacean Sacculina has been shown by Delage to
effect a partial castration of the crabs to which it fixes itself, and
the same has been observed by Giard in other cases. In two
such cases an approximation to the female form of appendage
has been observed. Rorig (1899) has shown that a diseased
state of the ovaries in deer is correlated with a development of
antlers, that atrophy of the testes is always followed by some
peculiarity in the antler-growth, that castration of a young
male always inhibits the development of antlers, and so on.
Sellheim (1898, 1899) finds that in many animals of both sexes
castration is followed by a prolongation of the period of bone-
growth. In the case of young cocks the effects of castration
are very variable, sometimes increasing, sometimes decreasing
the secondary sex characters. One result is clear, however,
that the whole body is affected; the larynx is intermediate in
size between that of cock and hen, the syrinx is weakly de-
veloped and the capons seldom crow or do so abnormally, the
brain and heart are light in weight, fat accumulates in the sub-
cutaneous and subserous connective tissue, and the skeleton
shows many abnormalities. The experiments of J. Th.
Oudemans (1898) on castrating caterpillars — a difficult opera-
tion — led him to the conclusion, in marked contrast to the
above, that there was little result either on the external appear-
ance or on the habits of the adults.
Thirdly, it should also be noted that in aged females, which
have ceased to be functional in reproduction, the minor pecu-
liarities of their sex often disappear, and they become liker
males, both in structure and habits, — witness the familiar case
of " crowing hens."
From the presupposition, then, of the intimate connection
between the sexuality and the secondary characters (which is
indeed everywhere allowed), it is possible to advance a step
further. Thus in regard to colour, that the male is usually
brighter than the female is an acknowledged fact. But pig-
THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 25
ments of many kinds are physiologically regarded as of the
nature of waste products. Such for instance is the guanin, so
abundant on the skin of fishes and some other animals.
Abundance of such pigments, and richness of variety in related
series, point to pre-eminent activity of chemical processes in the
animals which possess them. Technically expressed, abundant
pigments are expressions of intense metabolism. But pre-
dominant activity has been already seen to be characteristic of
the male sex; these bright colours, then, are often natural
to maleness. In a literal sense animals put on beauty for
ashes, and the males more so because they are males, and not
primarily for any other reason whatever. We are well aware
that, in spite of the researches of Krukenberg, Sorby, MacMunn,
Newbigin, and others, our knowledge of the physiology of
pigments is still very scanty. Yet in many cases, alike among
plants and animals, pigments are expressions of disruptive
processes, and are of the nature of waste products; and this
general fact is at present sufficient for our contention, that bright
colouring or rich pigmenting is more characteristic of the male
than of the female constitution.
In the same way, the skin eruptions of male fishes at the
spawning season seem more pathological than decorative, and
may be directly connected with the sexual excitement. One
instance of the way in which the reproductive maturity is known
to effect a by no means obviously related result may be given.
Every field naturalist knows that the male stickleback builds a
nest among the weeds, and that he weaves the material together
by mucous threads secreted from the kidneys. The little animal
is also known to have strong passions; it is polygamous in
relation to its mates, and most pugnacious in relation to its
rivals. Professor Mobius has shown that the male reproductive
organs (or testes) become very large at the breeding season, and
that they press in an abnormal way upon the kidneys. This
encroachment produces a pathological condition in the kidneys,
and the result is the formation of a mucous secretion, somewhat
similar to what occurs in renal disease in higher forms. To
free itself from the irritant pressure of this secretion, the male
rubs itself against external objects, most conveniently upon its
nest Thus the curious weaving instinct does not demand or
find rationale in the cumulative action of natural selection upon
an inexplicable variation, but is traced back to a pathological
and mechanical origin in the emphatic maleness of the organism.
afi THE EVOLUTION OF SEX.
The line of variation being thus given, it is of course coticeiv-
able thnt natural selection may have accelerated it.
So too, though again the physiological details are scanty, the
superabundant growth of hair and feathers may he interpreted,
in some measure through getting rid of waste products, for we
shall see later how local kataholistn favours cell multiplication.
Combs, wattles, and skin excrescences point to a predominance
of circulation in the skin of the feverish males, whose tempera-
tures are known in some cases to he decidedly higher than
those of the females. Even skeletal weapons like antlers may
be similarly interpreted; while the exaggerated activity of the
scent-glands is another expedient for excreting waste,
Hala (c), Worker <A), nnd Quran (a) Ant—From CAomiira'i £ncve., alUr Lnbbodc.
In regard to horns, feathers, and the like, in association with vigorous
drculalion, two senlcnees from Rolph may be quoted: — " The exceedingly
abundant circutalion, which perio<1]calty occurs in the at firat soft frantal
Kiotoberances of stags, admits and conditions the colossal development o(
orn and delicate ensheathing velvet. ... In the same way, (he rich
flow of blood in the feather papills conditions the immense growth of the
feathers, . . . end the same is true of hairs, s|Hncs, and teeth."
Professor J. Kennel gives expression in an interesting essay to an entirely
diSerenl interpiclation of such structures as antlers. It may be that they,
like the horns of Ruminants, were originally possessed bybolh sexes, and that
they have l>een lost by the females whose reproductive sacrifice leaves, as it
were, less to spare for such expensive structures as anilets are. .So it may
be thai the female deer have ceased to develop antlers except where the
conditions of life rendered their retention indispensable, namely, in the
reindeer. In other words, this may he one of the many cases in which the
female is nearer not to the ancestral but to the youthful type.
THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 2^
Some of the even subtler differences between the sexes are
of interest in illustrating the general antithesis. Thus in the
love-lights of the Italian glow insect {Lucioia\ the colour is
said to be identical in the two sexes, and the intensity is much
the same. That of the female, however, who is in other respects
rather male-like in her amatory emotions, is more restricted.
It is interesting further to notice that the rhythm of the light
in the male is more rapid and the flashes are briefer, while that
of the female is longer and the flashes more distant and tremu-
lous. This illustration may thus serve as a literally illumined
index of the contrasted physiology of the sexes.
The case of the Psychidse among Lepidoptera is particularly instructive.
Both males and females are normal caterpillars, but while the males become
winged, the female remains at the larval level, as regards absence of wings
and the usual adult appendages, oral as well as locomotor, but differing
from the larva in being reproductive. In short, the female degenerates to
the juvenile level except in productivity, while the male without doubt is
nearer the ancestral form.
§ 5. Sexual Selection: iis Limit as an Explanation. — We
are now in a better position to criticise Mr. Darwin's theory.
On his view, males are stronger, handsomer, or more emo-
tional, because ancestral forms happened to become so in a
slight degree. In other words, the reward of breeding-success
gradually perpetuated and perfected a casual advantage.
According to the present view, males are stronger, handsomer,
or more emotional, simply because they are males, — />., of
more active physiological habit than their mates. In phrase-
ology which will presently become more intelligible and
concrete, the males tend to live at a loss, are relatively more
kataboiic. The females, on the other hand, tend to live at a
profit, are relatively more anabolic, — constructive processes
predominating in their life, whence indeed the capacity of
bearing offspring.
No one can dispute that the nutritive, vegetative, or self-
regarding processes within the plant or animal are opposed to
the reproductive, multiplying, or species-regarding processes, as
income to expenditure, or as building up to breaking down.
But within the ordinary nutritive or vegetative functions of the
body, there is necessarily a continuous antithesis between two
sets of processes, — constructive and destructive metabolism.
The contrast between these two processes is seen throughout
nature, whether in the alternating phases of cell life, or of
28 THE EVOLUTION OF SEX.
activity and repose, or in the great antithesis between growth
and reproduction ; and it is this same contrast which we
recognise as the fundamental difference between male and
female. The proof of this will run through the work, but our
fundamental thesis may at once be roughly enunciated in a
diagrammatic expression (which in its present form we owe to
our friend Dr W. E. Fothergill) : —
Here the sum-total of the functions are divided into
nutritive and reproductive, the former into anabolic and
katabolic processes, the latter into male and female activities, —
so far with all physiologists, without exception or dispute.*
Our special theory lies, however, in suggesting the parallelism
of the two sets of processes. ' Thus maleness is associated
with a life-ratio in which katabolism has a relatively greater
SUM OF FUNCTIONS.
Nutrition. Reproduction.
Anabolism. Katabolism. Female. Male.
predominance than in the female. In terms of this thesis,
therefore, both primary and secondary sexual characters
express 'the fundamental physiological bias characteristic of
either sex. Sexual selection resembles artificial selection, but
the female takes the place of the human breeder; it resembles
natural selection, but the selective females and the combative
* The reader whose physiological studies may not have familiarised him
with that conception (really dating from Claude Bernard) of all physiological
processes as finding their ultimate expression in the metabolism (anal)olism
and katabolism) of protoplasm, will easily place himself in a position to
check our argument (often indeed, we trust to carry our interpretation of
sex into still fui^ther detail) by starting from the exposition of tnis doctrine
in Sir Michael Foster's article, ** Physiologv," in the Encychptedia
Britannica, or with Sir fiurdon Sanderson's Presidential Address to
Section D, British Association, 1889.
THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 29
males represent a role filled in the larger case by the fostering
or eliminating action of the environment. As a special case
of natural selection, Darwin's theory is open to the objection
of being teleological, i>., of accounting for structures in terms
of a final advantage. It is also open to the logical critic to
urge that the structures to be explained have to be accounted
for before, as well as after, the stage when they were developed
enough to be useful. The origin, or in other words the funda-
mental physiological import, of the structures must be ex-
plained before we have a complete or adequate theory of
organic evolution.
Apart from this logical insufficiency, the theory of sexual
selection is open to many minor objections, with some of
which Darwin himself dealt. One detailed objection which
seems serious may be noticed. The evolution of coloured
markings by selective preference carries with it the postulate
of a certain level of aesthetic taste and critical power in the
female, and this not only very high and very scrupulous as to
details, but remaining permanent as a standard of fashion from
generation to generation, — large assumptions all, and scarcely
verifiable in human experience. Yet we cannot suppose that
Mr Darwin considered the human female as peculiarly un-
developed. It is true, doubtless, that both insects and birds
have so far and increasingly become educated in such sensi-
tiveness; but when we consider the complexity of the mark-
ings of the male bird or insect, and the slow gradations from
one stage of perfection to another, it seems difficult to credit
birds or butterflies with a degree of aesthetic development
exhibited by no human being without both special aesthetic
acuteness and special training. Moreover, the butterfly, which
is supposed to possess this extraordinary development of
psychological subtlety, will fly naively to a piece of white
paper on the ground, and is attracted by the primary aesthetic
stimulus of an old-fashioned wall-paper, not to speak of the
gaudy and monotonous brightness of some of our garden
flowers. Thus we have the further difficulty, that we must
suppose the female butterfly to have a double standard of
taste, one for the flowers which she and her mate both visit,
the other for the far more complex colouring and markings of
the males. And even among birds, if we take those unmis-
takable hints of real awakening of the aesthetic sense which
are exhibited by the Australian bowerbird or by the common
30 THE EVOLUTION OF SEX.
jackdaw in its fondness for bright objects, how very rude is
this taste compared with the critical examination of in-
finitesimal variations of plumage on which Darwin relies. Is
not, therefore, his essential supposition too glaringly anthropo-
morphic ?
Again, the most beautiful males are often extremely com-
bative; and on the conventional view this is a mere coin-
cidence, yet a most unfortunate one for Mr Darwin's view.
Battle thus constantly decides the question of pairing, and in
cases where, by hypothesis, the female should have most
choice, she has simply to yield to the victor. On our view,
however, combative energy and sexual beauty rise pari passu
with male katabolism.
Or again, in the y£neas group of the genus Papilio^ Darwin
notes how there are frequent gradations in the amount of
difference between the sexes. Sometimes the sexes are alike
dull, where we should have to suppose the aesthetic perception
must somehow have been lost or inhibited; sometimes the
females are dull and the males splendid, — for Darwin, an
example of the result of sexual aesthetic perception, this of an
exquisitely subtle kind however, and without proportionate
cerebral enlargement In a third set of cases, both sexes are
splendid, which would suggest logically that the male in turn
had acquired a taste for splendour. But such cases, which
usually need more or less cumbrous additional hypothesis of
inheritance and so on to explain them, are intelligible enough
if we regard them as a relative increase of katabolism in the
life-ratio throughout a series of species. The third set may
be supposed to be relatively more katabolic than the first,
while the second set are midway; although, it may be freely
granted, a knowledge of the habits, size, &c., of the particular
species, would be necessary to verify the legitimacy of this
interpretation in each case.*
It is necessary once more to turn to the contrast between
the positions of Darwin and Wallace. According to Darwin,
sexual selection has accelerated the males into gay colouring;
* For a discussion of the progressive development of colouring and
markings, whether in butterflies or mammals, the reader may be referred
to the works of Professor Eimer, especially to his work on Lepidoptera.
Reference should also be made to Weismann's "Studies in the Theory of
Descent,'' for a discussion of the markings of caterpillars and butterflies.
THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 31
according to Wallace's original view, natural selection has
retarded the females (birds or butterflies) and kept them in-
conspicuously plain. It is no longer difficult to establish a
compromise. Both sexes have differentiated towards their
respective goals, not along lines of indefinite variation, but on
paths determined by the characteristic constitutions. In other
words, the secondary dimorphism has a definite physiological
basis in the primary sex-difference. If this interpretation of
the origin of the variants be granted, it remains a matter of
observation, experiment, and statistics to determine how far
the limits are fixed by natural selection (in Wallace's cases), or
by sexual selection (in Darwin's). The present position allows
the efficacy of natural and sexual selection as limiting, eliminat-
ing, in a sense directive, factors, but regards gay colouring as
the expression of the relative predominance of katabolism in
the male sex, and quiet plainness as equally natural to the pre-
dominantly anabolic females.
At a later stage something more will be said of natural
selection, and its limits as an explanation of facts. But it
is here desirable to emphasise, that just as we admit the
importance of sexual selection as a minor accelerant in the
differentiation of the sexes, so we are bound to recognise that
natural selection is also continually in operation as a check to
a divergence of the sexes which would otherwise tend to
become extreme. If this retarding influence of natural selec-
tion on the evolutionary process were not continually present,
we should find cases like Bonellia and the rotifers much
commoner than they are among animals. But it is an error
to exaggerate this limiting action into an explanation of the
process itself.
32 THE EVOLUTION OF SEX.
SUMMARY.
1-3. A broader basis must be sought from which to understand the
differences between the sexes. A general survey shows that the males are
more active in habit, the females more passive; that the males tend to be
smaller and to have a higher body-temperature, while the females tend to
be larger and to live longer.
4. The close association of secondary sexual characters with the re-
productive function is shown in the period or in the periodicity of their
development, in the effects of castration, in the peculiarities of aged
females, &c. Richer pigmentation, and other male characteristics, may
be interpreted as expressions of the relative katabolic predominance in the
constitution of males, as opposed to the relative anabolic preponderance of
the females.
5. Sexual selection, as an explanation of secondary sexual characters,
does not account for origins nor incipient stages, postulates subtle aesthetic
sensitiveness, and is beset by numerous minor difficulties. Yet the opposed
positions of Darwin and Wallace both emphasise indubitable facts; while
the criticisms of Mivart, the theory of Brooks, and the suggestions of
Rolph, Mantegazza, and others, lead on towards a deeper analysis. The
general conclusion reached, recognises sexual selection (so far with Darwin)
as a minor accelerant, natural selection (so far with Wallace) as a retarding
** lirake," on the differentiation of sexual characters, which essentially find
a constitutional or organismal origin in the relatively katabolic or anabolic
diathesis which characterises males and females respectively.
LITERATURE.
Brooks, Darwin, Mivart, Wallace.— As before.
Cunningham, J. T. — Sexual Dimorphism. London, 1900.
CURATOLO, G. E., and Tarulli, L. — Influence of the Removal of the
Ovaries on Metabolism. Edinburgh Med. Journal, 1895, PP* I37"'39'
Arch. ItaL Biol., XXXIII., 1895, PP- 388-390.
EiMBR, G. H. T. — Die Enstehung der Arten auf Grund von Vererben
erworbener Eigenschaften, nach den Gesetzen organischen Wachsens.
Jena, 1888.
Fowler, G. H. — Effects of Castration. Proc. Zool. Soc. London,
1894, pp. 485-494.
Gbddes, p. — Articles Reproduction, Sex, Vjiriation and Selection.
Encycl. Brit. Also, On the Theory of Growth, Reproduction, Sex,
and Heredity. Proc. Roy. Soc. Edin., 1885-86.
IIrring, E. — Theory of the Functions in Living Matter (1888). Trans.
by F. A. Welby. Brain, 1897, pp. 232-258.
Keller, C. A. — Evolution of (he Colours of North American Land-Birds.
p. (ialif. Acad., IIL, 1893, pp. 361, 19 pis.
I
K-*
THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 33
Kennel, J. — Stadien liber sexuellen Dimorphismus, Variation und ver-
wandte Erscheinungen. Schr. Nat. Ges. Dorpat, IX. (1896), p. 64.
OuDEMANS, J. Th.— Zool. Jahrb., XIL, 1898, pp. 71-88, 3 pis., 2 figs.
RoLPH, W. H. — Biologische Probleme. Leipzig, 1884.
RoussELET, C. F. — On the Male of Rhhiops vitrea, J. R. Micr. Soc,
part 116, 1897, pp. 4-9, I pi.
Sellheim, O. — Beitrage zur Geburtshilfo und Gynakologie, I. (1898),
pp. 229-246; and II. (1899), pp. 236-259.
Weismann, a. — Studies in the Theory of Descent (Meldola's Transla-
tion). London, 1880-82. The Germ- Plasm. London, 1893.
CHAPTER III
The Determination of Sex (Hypotheses and Observations),
So far the differences between the sexes as observed in adult
forms. Attention must now be turned to the origin of sex itself
in the individual organism. The historic beginning of sex will
be discussed at a later stage; the present problem concerns
the factors which determine whether any given organism will
develop into a male or into a female. The question, in other
words, is that usually known as the determination of sex.
§ I. The Period at which the Sex is Determined. — Every
organism, whether male or female, develops from a fertilised
egg-cell, apart of course from the occurrence of asexual and
parthenogenetic reproduction. This material, which in one
case develops into a male, in another into a female, is, so far
as our experience can go, always the same; 2Si^when the sex of
the organism is absolutely decided, is a question to which no
general answer can be given. In the higher animals (birds
and mammals) it is possible at quite an early date in embryonic
life to tell whether the young organism will turn into a male or
a female, though in the very earliest stages it is impossible to
determine whether the rudiment of the reproductive organs is
going to become a testis or an ovary. But in lower vertebrates,
such as frogs, the period of embryonic indifference is greatly
prolonged; and it seems certain that a hatched tadpole, even
after a tendency towards, say maleness, has actually arisen,
may in certain conditions have this altered in the opposite
direction. Among invertebrates, the sexual organs are often
late in acquiring definite predominance in favour of either sex, —
that is, the period of undecided indifference is, as one would
expect, usually much longer.
The factors which are influential in determining sex are
numerous, and come into play at different periods. The
constitution of the mother, the nutrition of the ova, the con-
stitution of the father, the state of the male element when
THE DETERMINATION OF SEX. 35
fertilisation occurs, the embryonic nutrition, and even the
laryal environment in some cases, these and yet other factors
have all to be considered.
Some observations by Laulani^ as to the embryonic organs are of
interest in this connection. He distinguishes both in birds and mammals
three stages in the individual development of the reproductive organs.
These he calls (i) Germiparity, (2) Hermaphroditism, (3) Differentiated
Unisexuality; and regards them as parallel to the stages of historic evolu-
tion. Even for the first stage, however, when the elements are still very
primitive, he would not allow the accuracy of the terms neutrality or
mdifierence. The elements in both sexes are almost similar, but yet their
future fate has been decided.
Sutton has also emphasised his conviction, that in the individual
development a state of embryonic hermaphroditism obtains, and main-
tains that one set of elements predominates over the other in the establish-
ment of the normal unlsexuaf state. Floss and others take up a similar
position in regard to an early hermaphrodite state. It can only be con-
cluded, that the higher the organism is in the series the earlier is its sexual
fate sealed ; and that it is only in lower vertebrates, and among backbone-
less animals, that we can speak of prolonged neutrality of sex, or embryonic
hermaphroditism.
§ 2. Answers to the Question: What Determines Sex? — To
the question what determines whether an organism shall develop
into a male or into a female, many and varied answers have
been given. At the beginning of the last century, the theories
of sex were estimated at as many as five hundred, and they have
gone on increasing. It is evident that even an enumeration of
these is not possible, nor is it indeed desirable. As in so
many other cases, our ideas respecting the determination of sex
have been looked at in three different ways. For the theologian,
it was enough to say that " God- made male and female." In
the period of academic metaphysics, still so far from ended, it
was natural to refer to " inherent properties of maleness and
femaleness;" and it is still a popular "explanation" to invoke
undefined " natural tendencies *' to account for the production
of males or females. This mode of treatment, it need not be
said, is being abandoned by biologists. It is recognised that
the problem is one for scientific analysis; thus the constitu-
tion, age, nutrition, and environment of the parents must be
especially considered. These investigations, which are mainly
restricted to observation and statistics, will be first noticed ; the
more experimental researches, and the general conclusions, will
be discussed in the next chapter. Finally, a physiological
36 THE EVOLUTION OF SEX.
re-Statement, in terms of protoplasmic metabolism, will be
suggested.
§ 3. The theory that there are two kinds of ova, respectively
destined to develop into males or females, is more than a mere
begging of the question. The constitution of the ovum is
undoubtedly a fact of primal importance, but we must also
recognise the results of experiment, which show that later
influences may also be determinative. The hypothesis of two
kinds of ova was advanced, for example, by B. S. Schultze, but
as the grounds for his views are not admitted as correct, only
its existence need be noticed till more observations are forth-
coming. Even if two kinds of ova were demonstrable, the
question would remain what conditions determine the pre-
dominance of this or the other kind. What is the biological
meaning of a family with seven daughters and one son ? What
is the biological meaning of Shufeldt's case of alternating sex
in the five young birds which formed the brood of a sparrow-
hawk ? The oldest was male, the next female, and so on in
regular alternation. ("Amer. Naturalist,'* xxxii., 1898, pp. 567-
57o» I fig-
§ 4. Numerous authors have attached great importance to
the process of fertilisation as a determinant of the sex.
One of the most crude positions has been that of Canestrini, who
ascribed the determination of sex to the number of sperms entering the
ovum : — The more sperms, the greater the tendency to male offspring. It
has, however, been shown by Fol, PflUger, Hertwig, and others, that
*' polyspermy," or the entrance of more than one sperm, is extremely rare,
is m fact generally impossible. In some of the cases where it is known to
occur, it indicates a pathological condition of the egg-shell, and tends to
produce abnormalities. PflUger diluted the seminal fluid of male frogs, and
found that no change resulted in the normal numerical proportion of the
sexes. The case of drones, furthermore, where males are known to arise
from unfertilised ova, is a ^miliar example, exactly counter to Canestrini's
proposition, which may in fact be dismissed as wholly untenable.
§ 5. Ttfft€ of Fertiiisaiion. — With greater weight various authorities
have insisted upon the time of fertilisation. Thus, according to Thury
(1863), followed by DUsing (1883), an ovum fertilised soon after liberation
tends to produce a female, while an older ovum will rather develop into a
male. As a practical breeder Thury claimed to determine the sex of cattle
upon this principle; Cornaz and Knight have both practically confirmed
this; while Girou has pointed out that female flowers fertilised as soon as
they were able to receive pollen tended to produce female of&pring.
Ilertwig has also shown that the internal phenomena of fertilisation vary
somewhat with the age of the ovum at the time. Hensen is inclined to
accept the general accuracy of Thury*s conclusion, but extends it to the
male element as well. *' A very favourable condition in both ovum and
THE DETERMINATION OF SEX.
37
sperm will probably lead to the formation of a female." ** Accordin^j to
its condition, a sperm may either insufficiently corroborate the favourable
state of the ovum, or constitutionally strengthen an ovum less satisfactorily
conditioned." A side-light is thrown on this by Vernon's experiments on
hybridising sea-urchins, which show that "the characteristics of the hybrid
offspring depend directly on the relative degrees of maturity of the sexual
products " (" Phil. Trans.," Series B, vol. cxc (1898), pp. 465-529). The
Summary of Statistics bearing on Relative Number of Males and Females.
Obierrcr.
No. of
Sixths.
Locality.
Father
older.
Proportion
of Males to
too Females.
Father of
equal aM.
Proportion
of Males to
inn Fa>inal<>f.
Father
younirer.
of Males to
100 Females.
Average
Propor-
ttOD of
Males to
too
Females.
Remarks.
Hofacker
1,996
Tilbingen
117.8
93.0
90-6
107.5
• •
Sadler
3,068
England
iai.4
94.8
86.5
114.7
■ •
G/ihlert
4,584
■ •
loS.
93-3
82.6
105.3
■ •
Legoyt
52,311
Paris
J04.49
102.14
97-5
103.97
■ •
Boulenger
6,006
Calais
109.98
107.92
10X.63
107.9
• •
Noirot
4,000
Dijon
99-7
• •
1 1 6.0
X03.5
• •
Breslau
8,084
Zftrich
103.9
103.1
117.6
106.6
• •
Stieda
100,590
Alsace-
Lorraine
105.03
• •
108.39
106.27
Contradictory.
Bemer
267,946
Sweden
104.61
106.23
107.45
ZO6.O
Contradictory
(sec text).
same observer has shown that the degree of staleness of the ova and sperms
of sea-urchins has an appreciable effect on the development (**P. Roy.
Soc.," Ixv. (1899), pp. 350-360).
§ 6. Age of Parents, — Hofacker (1823) and Sadler (1830) independently
published a body of statistics, each including about 2,000 births, in favour
of the generalisation that when the male parent is the older the offspring
are preponderatingly male; while if the parents be of the same age, or
38 THE EVOLUTION OF SEX.
a fortiori if the male parent be the younger, female offspring appear in
increasing majority. This conclusion, generally known as Hofacker's and
Sadler's law, has received both confirmation and perplexing contradiction.
It has been confirmed by Gohlert, Boulenger, Legoyt, and others, and by
some breeders of stock and birds, but is denied by other practical authori-
ties, and directly contradicted by the recent statistics of Stieda, from
Alsace-Lorraine, and of Berner, from Scandinavia.
The above table (in its upper part taken mainly from Hensen, afler
CEsterlen) shows vividly how much the results of Stieda and Berner conflict
with the law of Hofacker and Sadler. In regard to Berner's statistics, it
ought to be further noted that the figures quoted refer to cases where the
father or mother is only from i to 10 years the older. If the father be
more than ten years older, the male majority is 103.54; if the mother
be more than ten years older, the proportion is 104.10, again against
Hofacker*s and Sadler^s conclusion. Compared with the above human
statistics, Schlechter's results in regard to horses also militate against the
alleged law.
In regard to plants, various naturalists have drawn attention to the
influence of age upon sex. The following observations are quoted by
Heyer: — In Leontarus domestical according to Rumpf, the female plant
may bear male blossoms before its proper female flowers. In Morus nigra,
and in other cases, according to Miller, male flowers may be borne first,
and afterwards fruit. Treviranus observed that the first flowers of beech,
chestnut, and other trees are male. Clausen gives similar examples; and
Hoffmann notes that in the horse-chestnut, and several other cases, male
flowers appear first, and afterwards hermaphrodites or females.
Most of the results in regard to the influence of age are,
however, extremely unsatisfactory and conflicting. This is
evident from the above statistics. The law of Hofacker and
Sadler cannot be regarded as in any sense established. In
fact, as Hensen remarks, unless statistics are enormously large
they prove very little. The number of other factors besides
parental age which may operate in any case is evidently great,
— health, nutrition, frequency of sexual intercourse, abstinence
after the birth of a male, and the like, all reduce the feasibility
of the statistical method. At present, at any rate, we are not
justified in ascribing much importance to the relative age of
the parent except as a secondary factor, influential doubtless
in relation to nutrition.
§ 7. Comparative Vigour, — The best known, and probably
still most influential, theory is that of " comparative vigour."
As elaborated by Girou and others, this hypothesis connects
the sex of the offspring with that of the more vigorous parent.
It cannot be said, however, that facts bear out the case. Thus
consumptive mothers produce a great excess of daughters,
while Girou's theory would lead us to expect the opposite.
THE DETERMINATION OF SEX. 39
We require in fact to have *' vigour" analysed out into its
component factors, and in so doing we shall afterwards find
not only facts but reasons in favour of the conclusion, in part
included in the above theory, that highly nourished females
tend to produce female offspring. That form of the hypo-
thesis which refers the determination of sex to ** genital
superiority," or to " relative ardency," can hardly be seriously
considered. In this connection it has been maintained that in
" marriages of love," after a short betrothal, female offspring
predominate ; and a number of other interesting facts of a
like nature are suggested. Some scepticism as to the practi-
cability of such inductions is, however, inevitable.
§ 8. Starkweather^ s Law of Sex, — Closely allied to the
theory of comparative vigour is that elaborately worked out by
Starkweather, which is suggestive enough to deserve separate
summary. He starts from a discussion of the alleged superi-
ority of either sex. Few maintain that the sexes are essen-
tially equal, still fewer that the females excel; the general
bias of authority has been in favour of the males. From the
earliest ages philosophers have contended that woman is but
an undeveloped man ; Darwin's theory of sexual selection
presupposes a superiority and an entail in the male line;
for Spencer, the development of woman is early arrested by
procreative functions. In short, Darwin's man is as it were
an evolved woman, and Spencer's woman an arrested man.
This notion of the superiority of males has formed the
basis of many theories of sex. As a good illustration of this
opinion, a few sentences may be quoted from Richarz: — "The
sex is not a quality transmitted from the parents, but has its
basis in the degree of organisation attained by the offspring.
The male sex represents to a certain extent a higher grade of
organisation or development in the embryo. This is attained
when the reproductive efficiency of the mother is specially
well developed, and the resulting male offspring more or less
resembles the mother. But if the maternal reproductive
power be weak, the ovum does not attain to maleness, and the
resulting female offspring more or less resembles the father."
Thus Hough thinks males are born when the maternal system
is at its best ; more females at periods of growth, reparation,
or disease. Tied man and others regard female offspring as
arrested in the original state ; while Velpau conversely regards
females as degenerate from primitive maleness.
40 THE EVOLUTION OF SEX.
Reacting from such speculations as to superiority of either
sex, Starkweather firmly maintains that "neither sex is
physically the superior, but both are essentially equal in a
physiological sense." This is true in the average, but yet in
each pair a greater or less degree of superiority on one side or
other must usually be conceded. Granting this, Starkweather
states, as his chief conclusion, " that sex is determined by the
superior parent, also that the superior parent produces the
opposite sex." Referring the reader to the Ency. Brit. Article
** Sex," for some critical notes, it is enough here to notice, that
just like "comparative vigour," so "superiority" has little
more than verbal simplicity to recommend it, since it lumps a
great variety of factors under a common name. Yet, in justice
to its author, we may admit that it is the algebraic sum of
these which he aims at expressing.
§ 9. DarwirCs Position, — Neither in regard to the origin of
sex, nor its determination in individual cases, did Darwin see
further than his contemporaries. He refers to the current
theories of the influence of age, period of impregnation, and
the like ; and further contributes a great body of statistics on
the numerical proportions of the sexes, and the supposed
influence of polygamy. "There is reason," he says, "to
suspect that in some cases man has by selection indirectly
influenced his own sex-producing powers." He falls back
upon the unanalysed " belief that the tendency to produce
either sex would be inherited like almost every other peculiarity,
for instance, that of producing twins." " In no case, as far as
we can see, would an inherited tendency to produce both sexes
in equal numbers, or to produce one sex in excess, be a direct
advantage or disadvantage to certain individuals more than to
others ; . . . and therefore a tendency of this kind could
not be gained through natural selection." " I formerly thought
that when a tendency to produce the two sexes in equal
numbers was advantageous to the species, it would follow from
natural selection, but I now see that the whole problem is so
intricate that it is safer to leave its solution for the future."
Any other hints that Darwin threw out, have been so well
elaborated by Dusing's work on the advantageous self-regula-
tion of the sex-proportions, that reference to the latter is more
profitable.
§ 10. Diising on the Proportions of the Sexes ^ atid the
Regulation of these, — In an important work, Diising has
/
THE DETERMINATION OF SEX. 4I
recently treated the whole subject with some synthetic result.
He recognises that the fates or factors determining the sex are
manifold, and operate at different periods. Much is determined
by the condition of the reproductive elements — />., by the con-
stitution and habits of the i>arents ; much depends also on the
period of fertilisation ; while again the nutrition of the embryo
may be of moment. Diising has collected a great body of
facts, from both plants and animals, in favour of his conclu-
sions; but the copious summary of his work, given in the article
" Sex " already referred to, need not here be repeated, while
some of his experimental results will be included in the next
chapter.
Diising's memoir is very important, however, for this special
reason, that he analyses what may be termed the mechanism
by which the proportion of the sexes is regulated. Instead of
vaguely referring the whole matter to natural selection, he
shows in detail how the numbers are in a sense self-regulating,
how there is always produced a majority of the sex that is
wanted. That is to say, if one sex be in the decided minority,
or under conditions which come to the same thing, then a
majority of that sex will be produced. If there be, for instance,
a great majority of males, there is the greater likelihood of the
ova being fertilised early, but that means a probable pre-
ponderance of female offspring, and thus the balance is
restored. It would be rash to say that in every case he
makes out his contention, but his general argument, that
disturbances in the proportion of the sexes bring about
their own compensation, is carefully and convincingly worked
out.
§ II. Sex of Twins, — It sometimes happens among many different
classes of animals that from one ovum two organisms develop. We have
then a case of ** true " twins, as opposed to cases where multiple offspring
do not arise from one ovum. Such "true" twins are said to occur not
uncommonly in the human species, and are either most markedly similar
to one another or strongly dissimilar.
From a very early date an exception to this rule has been known in
regard to cattle, and applies to some other organisms as well. From the
careful researches of Spiegelberg and others, it appears that in cattle {a)
the twins may be both female and then both normal, or {b) that the sexes
may be difl'erent and normal, or (c) that lx>th may be males, in which case
one always exhibits the peculiar abnormality known as a *' free-martin."
The internal organs are male, but the external accessory organs are female,
and there arc also rudimentary female ducts. No theory has yet explained
the facts of this case.
42 THE EVOLUTION OF SEX.
It is now necessary, with Diising for transition, to pass from
the historical mode of treatment to something more con-
structive. Leaving mere hypotheses behind, as well as theories
based on insufficient statistics, an induction from experimental
evidence will be built up in the following chapter.
THE DETERMINATION OF SEX. 43
SUMMARY.
1. The epoch at which the sex is finally determined is variable in
different animals, and diverse factors operate at successive epochs.
2. Theological and metaphysical theories of sex have preceded the
scientific; observation and statistics have been resorted to l^efore experi-
ment; and over 500 theories in all have been set forth.
3-6. That there are two kinds of ova is still for the most part an
assumption; that the entrance of more than one spermatozoon normally
occurs, and is a determining factor, is erroneous. Thury's emphasis on the
age of the ovum when fertilised is probably justified ; while Hensen extends
this notion to the male element as well. The age of the parents is probably
only of secondary import, and the law of Hofacker and Sadler is not
confirmed.
7, 8. Theories of ** comparative vigour " and the like must he dis-
missed ; while Starkweather s theory of the relative superiority of either
sex, and of the influence of this on the sex of the offspring, requires further
analysis.
9, 10. Darwin's position contains nothing novel, and has been superseded
by Diising's synthetic treatment and explanation of the self-regulating
numerical proportion of the sexes.
II. From this point, after a note on the similar sex of "true" twins,
we pass to the experimental data and constructive treatment.
LITERATURE.
Brrner. — Hj. Om Kjonsdannelsens Aarsager, En biologisk Studie (with
numerous references). Christiania, 1883.
Born, G. — Experimentelle Untersuchungen Uber die Entstehung der
Geschlechtsunterschiede. Breslauer aerztliche Zeitschrift. 1881.
C1.BISZ, A. — Recherches des lois qui president k la creation des Sexes.
Paris, 1899, pp. 81 (with bibliography).
CoH^^, L. — Die willkiirliche Bestimmung des Geschlechts. 2nd ed.
Wurzburg, 1898.
Darwin, C. — The Descent of Man, Chap. VIII. London, 1871.
The Variation of Animals and Plants under Domestication. Lond.
DusiNG, C. — Die Regulierung des Geschlechtsverhiiltnisses bei der
Vermehrung der Menschen, Thiere, und Pflanzen. Jena, 1884; or,
Jen. Zeitsch. f. Naturw., XVII., 1883.
Gkddks, p.— As before.
GiROU DE BuzARRiNGUES.— De la generation. Paris, 1828.
Hknneberg, B. — Wodurch wird das Geschlechtsverhaltnis beim Men-
schen und den hoheren Tiercn beeinflusst. Anatomische Ergebnisse
(Merkel and Bonnet), VII., 1897, pp. 697-721.
Hbnsbn, V. — Physiologic der Zeugung. Hermann's Handbuch der
Physiologic, Bd. VI., pp. 304, with references to Ploss, Schultze, &c.
Leipzig, 1 88 1.
His, W. — Theorien der geschlechtlichen Zeugung. Arch. f. Anthropologic,
Bde, IV.. VI.
44 THE EVOLUTION OF SEX.
HoFACKER. — Ueber die Eigenschaften, welche sich bei Menschen und
Thieren auf die Nachkomnien vererben. Tubingen, 1828.
Jankr, H.— Die willkUrliche Hervorbringung des Geschlechts. 2nd ed.
Berlin, 1888.
Laulani^, F.— Comptes Rendus, CL, pp. 593-5. 1885.
Roi.PH, W. H.-— As before.
Roth, £. — Die Thatsachen der Vererbung (historical). Berlin, 1885.
PflDger, E. — Ueber die das Geschlecht bestimmenden Ursachen und die
Geschlechts - verhaltnisse der Frosche. Arch. f. d. ges. Physiol., a
XXIX., 1882.
Sadlbr. — The Law of Population. London, 1830.
ScHBNK, L. — Einfluss auf das Geschlechtsverhaltnis. Magdeburg, 1898.
ScHLBCHTBR. — Ueber die Ursachen welche das Geschlecht bestimmen.
Rev. f. Tierheilkunde, Wien, 1884. Biol. CentralblU, IV., pp. 627-9.
Sbligson, E. — ^WillkUrliche Zeugung von Knaben oder Madchen. MUn-
chen, 1895.
Stark WBATH BR. — The Law of Sex. London, 1883.
Stirda. — Das Sexual Verhaltniss bei Geborenen. Strasburg, 1875.
Sutton, J. B. — General Pathology. London, 1886.
Thury. — Ueber des Gesetz der Erzeugung der Geschlechter. Leipzig,
1863.
Wappceus. — Allgemeinc Bevolkerungs-Statistik. Leipzig, 1861. 1
Watase. — On the Phenomena of Sex-Differentiation. Journal of Mor-
phology, 1892. '<!
WiLCKENS, M.~Untersuchungen ubcr das Geschlechtsverhaltnis. Berlin, |
1886. '
CHAPTER IV.
The Determination of Sex.
{Experiment and Rationale.)
§ I. Influence of Nutrition, — Throughout nature the influence
of food is undoubtedly one of the most important environ-
mental factors. To Claude Bernard, indeed, the whole problem
of evolution was very much a question of variations of nutrition.
" L'evolution, c'est Tensemble constant de ces alternatives de
la nutrition ; c'est la nutrition consider^e dans sa r^alit^, em-
brass^e d'un coup d'oeil k travers le temps." It is fitting that
we should begin our survey of the factors known to influence
sex with the fundamental function of nutrition.
{a) TIu Case of Tadpoles, — Not a few investigators who
have passed from statistics and hypothesis to experiment and
induction, have found their material in tadpoles, where the sex
seems to remain for a comparatively long period indeterminate.
If we take the verdict of Yung, who has had much experience
with these forms, tadpoles pass through a hermaphrodite stage,
in common, according to other authorities, with most animals.
During this phase external influences, and especially food, decide
their fate as regards sex, though the hermaphroditism, as we
shall afterwards see, sometimes persists in adult life. It is fair,
however, to notice that Pfliiger gives a somewhat diflerent
account of the actual facts, distinguishing among tadpoles three
varieties — (a) distinct males, (b) distinct females, and (i) herma-
phrodites. In the last, testes, or male organs, develop round
primitive ovaries, and if the tadpoles are to become males the
enclosed female organs are absorbed.
Adopting the view stated by Yung, we shall simply state the
striking results of one series of observations. When the tadpoles
were left to themselves, the percentage of females was rather in
the majority. In three lots, the proportions of females to
males were as follows : — 54 : 46 ; 61 : 39 ; and 56 144. The
average number of females was thus about 57 in the hundred.
46 THE EVOLUTION OF SEX.
In the first brood, by feeding one set with beef, Yung raised the
percentage of females from 54 to 78 ; in the second, with fish,
the percentage rose from 61 to 81 ; while in the third set, when
the flesh of frogs was supplied, the percentage rose from 56
to 92. That is to say, in the last case the result of altered
diet was that there were 92 females to 8 males. From the
experience and carefulness of the observer, these striking
results are entitled to great weight
(i) Case of Bees. — The three kinds of inmates in a beehive
are known to every one as queens, workers, and drones ; or, as
fertile females, imperfect females, and males. What are the
factors determining the differences between these three forms?
In the first place, it is believed that the eggs which give rise to
drones are not fertilised, while those that develop into queens
and workers have the normal history. But what fate rules the
destiny of the two latter, determining whether a given ovum
The Queen (a), Worker (c), Md Dnne (b)
oriheComnKHiH've-liH.
will develop into the possible mother of a new generation, or into
the better- brained but non-fertile working female } It seems
certain that the fate mainly lies in the quantity and quality of
the food. Royal diet, and plenty of it, develops the reproductive
organs of the future queens ; sparser and plainer food retards
the sexuality of the future workers, in which reproductive organs
do not develop. Up to a certain point, the nurse bees can
THE DETERMINATION OF SEX.
47
determine the future destiny of their charge by changing the
diet, and this in some cases is certainly done. If a larva on
the way to become a worker receive by chance some crumbs
from the royal superfluity, the reproductive function may develop,
and what are called "fertile workers," to a certain degree above
the average abortiveness, result; or, by direct intention, a worker
grub may be reared into a queen bee.
The following table, after a recent analysis by A. von Planta, shows the
differences of diet as far as solids are concerned. For queens 69.38 per
cent., for drones 72.75 per cent., and for workers 71.63 per cent, is water.
Solids.
Nitrogenous
Fatty
Glucose ..
Ashes
Queens.
45-14
1355
«o.39
4.06
Drones.
I to 4 days.
Drones.
After 4 days.
Workers.
55- 9 »
11.90
9-57
31.67
4-74
38.49
2.02
51.21
6.84
27.65
From the above, it is seen that the queen larvx get a quantity of fatty
material double that given to the workers. The drones at first receive a
large percentage of nitrogenous material, but this soon falls below the
share which workers and aueens obtain. The fatty material, at first
large, soon falls to about a third of that given to the queens. Hence the
percentage of glucose, except at first, is so much larger than in the other
two cases.
It is not necessary, however, to go into details to see the
importance of the main point, that differences of nutrition, in
great part at least, determine the all-important distinctions
between the development and retardation of femaleness. Nor
are there many facts more significant than this simple and well-
known one, that within the first eight days of larval life, the
addition of a little food will determine the striking structural
and functional differences between worker and queen.
Eimer has drawn attention to the interesting correlation ex-
hibited in the fact that a larva destined to become a worker,
but converted into a queen, attains with the increased sexuality
all the little structural and psychological differences which
otherwise distinguish a queen. Regarding fertilisation as a sort
of nutrition, he considers drones, workers, and queens as three
terms of a series, and the same view is suggested by Rolph.
Eimer recalls some interesting corroborations from humble bees.
There the queen mother, awakened from her winter sleep by
the spring sun, makes a nest, collects food, and lays her first
48
THE EVOLUTION OF SEX.
brood. These are not too abundantly supplied with nourish-
ment, the queen having much upon her shoulders ; they develop
into small females, workers in a sense, but yet fertile, though
only to the extent of producing drones. By-and-by a second
brood of workers is born ; these have the advantage of the
existence of elder sisters, are more abundantly nourished, and
develop into large females. Still, like the first brood, they pro-
duce drones, though occasionally females. Finally, with the
advantage of two previous broods of small and large females,
the future queens are bom. The above facts not only afford
an interesting corroboration of the influence of nutrition upon
sexuality, but are of importance as suggesting the origin of the
more highly specialised society of the hive bee.
{c) Von SieboliTs Experiments, — With a somewhat different purpose
than that at present pursued , Von Siebold made a series of careful observa-
tions on a species of wasp, NemcUus ventricosus. These afford, as Rolph
has noted, some valuable results in regard to the determination of sex. In
this wasp, the fertilised ova, unlike those of hive bees, develop into males
as well as females; while the unfertilised, or parthenogenetic eggs, mav pro-
duce females in small percentage. From spring onwards, as warmth and
food both increased, Von Siebold estimated the percentages of males and
females in broods of larvae reared from fertilised ova. The results of a
series of observations may be condensed in a table : —
End op Larval Period
(Pupation).
No. of
Females
to xoo Males.
No. of
Females.
Naof
Males.
15th June
]t :: :: ::
Auffust
End of August
September
14
77
269
340
500
xoo
579
• •
« •
136
66
• •
• •
• •
As Rolph remarks, the results are not altogether satisfactory for the
present purpose, ** but this much is clear, that the percentage of females in-
creases from spring to August, and then diminishes. We may conclude
without scruple, that the production of females from fertilised ova increases
with the temperature and with the food supply {AssimilatiansUistung^
and decreases as these diminish."
From the work of Rolph, which is full of a suggestiveness which the
author unfortunately did not live to elaborate, we shall quote another
paragraph summing up further experiments of Von Siebold : —
** Not less instructive," he says, "are the experiments with unfertilised
ova (see Table).
'*This table shows the same general result as before. The more
abundant the metabolism [Stoffwechsel) and the nutrition, the greater
THE DETERMINATION OF SEX,
49
tendency to the production of females, which at the beginning and at the
end are wholly absent. In the above series of experiments, they only
appear when the metabolism and the nutrition were so abundant
that the entire development of the young wasps only occupied eighteen or
No. of
Dunitioii orr'nibryonic
Sex.
Experiment 1
and Larval State.
ZI
21 days
All Males.
la
»9 ..
All Males.
X3
18 .,
493
Males. 2 Females.
M
17 »
263
»» 3 ,,
15
'I "
374
M 8 ,,
16
18 „
168
u ' »»
17
34 >.
I
11 • •
fewer days up to the period of pupation." The peculiarity in this last case,
if the experiments were correct, is that in parthenogenesis, where the
production of males is the normal condition, favourable environmental
influences anuear to introduce females.
Two Fonns of a Common Plant-Louse or Aj^!s.>-
This figure may serve to illustrate three different
things, — a winged male and a wingless female; a
winged and a wingless parthen(^enetic female; a
winged sexual female, and an ordinary winglesi
parthenogenetic female.— From Kessler.
(d) Case of Aphides, — One of the most familiar illustrations
of the influence of nutrition upon sex, is found in the history
of -the plant-lice or aphides, which is indeed full of other
suggestions in regard to the whole theory of sex and reproduc-
tion. Details in regard to these plant-lice, which multiply so
rapidly upon our rose-bushes, fruit-trees, and the like, differ
4
50 THE EVOLUTION OF SEX.
somewhat in the various species, but the general facts are
recognised to be as follows. During the summer months,
with favourable temperature and abundant food, the aphides
produce parthenogenetically generation after generation of
females. The advent of autumn, however, with its attendant
cold and scarcity of food, brings about the birlh of males, and
the consequent recurrence of strictly sexual reproduction. In
the artificial environment of a greenhouse, equivalent to a
perpetual summer of warmth and abundant food, the partheno-
genetic succession of females has been experimentally observed
for four years, — it seems in fact to continue until lowering of
the temperature and diminution of the food at once re-intro-
duce males and sexual reproduction.
{e) Butterflies atid Moths, — Still keeping to insects, we may
note Mrs Treat's interesting experiment, that if caterpillars
were shut up and starved before entering the chrysalis state
the resultant butterflies or moths were males, while others of
the same brood highly nourished came out females. Gentry
too has shown for moths that innutritious or diseased food
produced males, and suggests this as a partial explanation of
the excess of male insects in autumn, although we suspect
that temperature is in this instance probably more important.
It should be noted, however, that Paulton's experiments on
the sexes of larvae of Smerinthus populi give no support to the
conclusion that the sex can be determined by external con-
ditions during larval life. The larger female larvae require
more food, and when supplies are reduced they tend to starve
first ("Trans. Entomol. Soc. London," 1893, pp. 451-6).
(/) Crustaceans, — In support of the same contention,
Rolph has drawn attention to the following among other facts.
One of the brine shrimps (Artemia saiina) resembles not a
few crustaceans in the local and periodic scarcity or absence of
males, associated of course with parthenogenesis. At Mar-
seilles, Rolph says, this artemia lives in especially favourable
conditions, as its large size plainly indicates ; there it produces
only females. Where the conditions of existence are less
prospeious, it produces males as well. "A certain maximum
of abundance and optimum of vital conditions in partheno-
genetic animals — daphnids and aphides, Apus, Branchipus,
Artemia, and numerous other crustaceans — produce females ;
while less favourable conditions are associated with the pro-
duction of males." In regard, however, to water-fleas
THE DETERMINATION OF SEX. 5 1
(daphnids), it is fair to notice that Rolph's conclusions do
not quite consist with Weismann's, who, with unique experi-
ence in regard to these curious little animals, is disinclined to
allow the direct influence of temperature and nutrition in the
matter.
{£) In regard to Rotifers {Hydatina\ Maupas maintains
that temperature is the sex-determining factor, and that the
sex of the offspring is determined two generations in advance !
His experiments led him to conclude that when the ovum is
being differentiated in the ovum, the temperature determines
whether it shall develop into a male-producing or a female-
producing individual Nussbaum, on the other hand, disputes
the conclusiveness of this result, and maintains that nutrition
is the determining factor : females of Hydatina which have
been insufficiently fed during early life afterwards lay only male
eggs, while well-nourished forms produce female eggs.
(A) Mammals, — ^When we pass to higher animals, the diffi-
culties of proving the influence of nutrition upon sex are much
greater. Yet there are decisive observations which go to
increase the cumulative evidence. Thus an important experi-
ment was long ago made by Girou, who divided a flock of
three hundred ewes into equal parts, of which the one-half
were extremely well fed and served by two young rams, while
the others were served by two mature rams and kept poorly
fed. The proportion of ewe lambs in the two cases was re-
spectively sixty and forty per cent In spite of the combination
of two factors, the experiment is certainly a cogent one.
Diising brings forward further evidence in favour of the same
conclusion, noting, for instance, that it is usually the heavier
ewes which bring forth ewe lambs. He emphasises the fact
that the females having a more serious reproductive sacrifice,
are more dependent on variations of nutrition than males.
Even in birds, as Stolzmann points out, there is a much greater
flow of blood to the ovary than to the testes, — the demands are
greater, and the consequences therefore more serious if these
are not fulfilled.
(/) In the human species, lastly, the influence of nutrition,
though hard to estimate,^ is more than hinted at. Ploss may
be mentioned as an authority who has emphasised this factor
in homo. Statistics seem to show, that after an epidemic or a
war the male births are in a greater majority than is usually
the case. Diising also points out that females with small
52 THE EVOLUTION OF SEX.
placenta and little menstruation bear more boys, and contends
that the number of males varies with the harvests and prices.
In towns, and in prosperous families, there seem to be more
females, while males are more numerous in the country and
among the poor.
Schenk has (1898) re-enunciated the view that nutrition is
the chief determining factor in deciding the sex of offspring.
But his evidence is quite insufficient; indeed, when it is
critically examined it is seen to consist of three or four cases.
(j) Determination of Sex in Plants. — It is at present ex-
tremely difficult to come to any very satisfactory conclusion in
regard to the influence of nutrition upon the sex of plants.
The whole subject, as far as its literature is concerned, has
been recently discussed by Heyer, but his survey is by no
means a sanguine one. His conclusions, in fact, seem to land
him in a scepticism as to all modification of the organism by
environmental influences, which we should of course be far
from sharing. It must be admitted that the experiments of
Girou (1823), Haberlandt (1869), and others, yielded no cer-
tain result ; while the conclusions of some others are conflict-
ing enough to justify not indeed Heyer*s despair, but his
present caution. Still a few investigations, especially those of
Meehan (1878), which are essentially corroborated by Diising
(1883), go to show, for some cases, that abundant moisture
and nourishment do tend to produce females. Some of
Meehan*s points are extremely instructive. Thus old branches
of conifers, overgrown and shaded by younger ones, produce
only male inflorescence. In the American Corylus rostrata,
and in many other instances, he is convinced that in early
stages the sex of a flower-bud is undetermined, and that its
determination as a male or female flower is mainly the result
of the nutritive conditions ("Proc. Acad. Nat. Sci. Phila-
delphia," 1899, pp. 84-86). Various botanists, quoted by
Heyer, confirm one another in the observation that prothallia
of ferns grown in unfavourable nutritive conditions prpduce
only antheridia (male organs), and no archegonia (female
organs).
The botanical evidence, though by no means strong, cor-
roborates the general result that good nourishment produces a
preponderance of females. The contrast of the sexes in some
of our common dioecious plants is here very instructive.
Taking for instance the dog-mercury {Mercuriaiis perennis) of
THE DETERMINATION OF SEX. 53
any shady dell, or the day lychnis (Z. diurna\ hardly less
abundant on the sunnier slopes, experiments are still certainly
wanting with regard to given plants, as to what circumstances
originally determined their sexual differences ; but the fact of
superior constitutional vegetativeness in the females is here so
peculiarly obvious, that it can hardly fail to arouse a strong
impression that more or less advantageously nutritive con-
ditions, whether of the embryo or of the seedling, are sufficient
to account for the differences of sex.
§ 2. Influence of Temperature. — In this connection not a
few writers have referred to an observation by Knight, which,
from its comparatively ancient date, perhaps deserves to be
recorded in his own words, if only to show the necessity of
caution in such matters. A water-melon was grown in a heated
glass-house, where the temperature sometimes rose on warm
days to 110'' Fahr. " The plant grew with equal health and
luxuriance, and afforded a most abundant blossom; but all its
flowers were male. This result did not in any degree surprise
me, for I had many years previously succeeded, by long con-
tinued very low temperature, in making cucumber plants
produce female flowers only; and I entertain but little doubt
that the same fruit stalks might be made, in this and the
preceding species, to support either male or female flowers in
obedience to external causes."
This experiment was obviously more sanguine than satis-
factory. Heyer justly points out that of the water-melon only
a single plant was taken. Furthermore, he says, the water-
melon in nature usually bears only female flowers on the apices
of the older twigs, and may bear only a minimum number of
these. Knight's observations on cucumbers are also open to
serious objections, and were too scanty to prove anything.
Meehan finds that the male plants of hazel grow more
actively in heat than the female; and Ascherson has made the
interesting observation that the water-soldier (Stratiotes aloides)
bears only female flowers north of 52* lat, and from 50' south-
wards only male ones. On the other hand, Molliard maintains
("Comptes Rendus," cxxvii., 1898, pp. 669-671) that in the
case of dog's mercury {Mercurialis annua) a high temperature
favours the production of female individuals, but whether the
heat simply promotes especially the development of the female
seeds, or has some direct eflect on the nature of the seed, is
left undetermined. The same experimenter maintains in regard
54 THE EVOLUTION OF SEX.
to the hop that the sex is not absolutely determined in the
seed, and that a transformation may be observed from male to
female inflorescences under conditions that are very unfavour-
able to the development of the vegetative organs, e,g.^ feeble
illumination.
In the human species, Diising and others have noted that
more males are born during the colder months; and Schlechter
has reached the same results from observations upon horses.
The temperature of the time, not of birth but of sex determina-
tion, is however more important; nor must it be forgotten that
temperature may have many indirect and subtle influences.
§ 3. Summary of Factors, — If we now sum up the case, it
must flrst be recognised that a number of factors co-operate in
the determination of sex ; but that the most important of these
may be more and more resolved into plus or minus nutrition,
operating upon parent, sex elements, embryo, and in some
cases larvae.
(a) Starting with the parent organisms themselves, we And
this general conclusion most probable, — that adverse circum-
stances, especially of nutrition, but also including age and the
like, tend to the production of males, the reverse conditions
favouring females.
{b) As to the reproductive elements, a highly nourished
ovum, compared with one less favourably conditioned, in every
probability will tend to a female rather than to a male develop-
ment. Fertilisation, when the ovum is fresh and vigorous,
before waste has begun to set in, will corroborate the same
tendency.
{c) Then if we accept Sutton's opinion as to a transitory
hermaphrodite period in most animals, from which the transition
to unisexuality is effected by the hypertrophy of the female side
or preponderance of the male in respective cases, the vast
importance of early environmental influences must be allowed.
The longer the period of sexual indifference (though this term
be an objectionable one) continues, the more important must
be those outside factors, whether directly operative or indirectly
through the parent. Here again, then, favourable conditions
of nutrition, temperature, and the like, tend towards the pro-
duction of females, the reverse increase the probability of male
preponderance.
The general conclusion, then, more or less clearly grasped
by numerous investigators, is that favourable nutritive con*
THE t>ETERMiNAtlON OF SEk. 5^
ditions tend to produce females, and unfavourable conditions
males.
§ 4. Let us express this, however, in more precise language.
Such conditions as deficient or abnormal food, high tempera-
ture, deficient light, moisture, and the like, are such as tend to
induce a preponderance of waste over repair, — a relatively kaia-
bolic habit of body, — and these conditions tend to result in the
production of males. Similarly, the opposed set of factors,
such as abundant and rich nutrition, abundant light and mois-
ture, favour constructive processes, i.e.^ make for a relatively
anabolic habit, and these conditions tend to result in the pro-
duction ol females. With some element of uncertainty, we
may also include the influence of the age and physiological
prime of either sex, and of the period of fertilisation. But the
general conclusion is tolerably secure, — that in the determina-
tion of sex, influences inducing a relative predominance of
katabolism tend to result in production of males, as those
favouring a relative predominance of anabolism similarly in-
crease the probability of females.
§ 5. This is not all, however; the above conclusion is in-
deed valuable, but it acquires a deeper significance when we
take it in connection with the result of a previous chapter.
There it was seen, as the conclusion of an independent induc-
tion, that the males were forms of smaller size, more active
habit, higher temperature, shorter life, &c.; and that the females
were the larger, more passive, vegetative, and conservative forms.
Theories of "inherent" maleness or femaleness were rejected,
since practically merely verbal ; more accurately, however, they
have been interpreted and replaced by a more material con-
ception, which finds the bias of the whole life, the resultant of
its total activities, to be a predominance of the protoplasmic
processes either on the side of disruption or construction.
This conclusion has still to receive cumulative proof, but one
large piece of evidence is now forthcoming, that, namely, of
the present chapter. If influences favouring katabolism make
for the production of males, and if anabolic conditions favour
females, then we are strengthened in our previous conclusion,
that the male is the outcome of relatively predominant kata-
bolism, and the female of relatively predominant anabolism.
§ 6. WeismanrCs Theory of Heredity, — In thinking of the
environment as a factor determining the sex, it is impossible to
ignore that such facts as we have noted above have some
§6 I'riE EVOLUTION OF SEJ(.
bearing upon the problem of heredity. Much of the recent
progress in the elucidation of the facts of inheritance has been
due to Weismann, who, in his theory of the continuity of the
germ-plasm, has restated the very important and fundamental
conception of a continuity between the reproductive elements
of one generation and those of the next. To this restatement
we shall afterwards have to refer; it is with another position,
not peculiar to, but emphasised by the same authority, that we
have here to do, viz., with his denial of the inheritance of
individually acquired characters. Any new character exhibited
by an organism may arise in one of two ways, which it is easy
enough to distinguish theoretically; — it may be an outcrop of
some property inherent in the fertilised egg-cell, that is, it may
have a constitutional or germinal origin; but, on the other
hand, it may be impressed upon the individual organism by the
environment, or acquired in the course of its functioning, that
is, it may have a functional or environmental origin. But all
such functional and environmental modifications are, according
to Weismann, restricted to the individual organism; they are
not transmissible.
In this denial of the transmission of dints from without,
and of acquired habits other than constitutional, Weismann
expresses a scientific scepticism, based on the one hand on the
absence of data demonstrating what we may still call the
current belief, and on the other hand on the improbability of
modifications reacting from the "body" on the reproductive
cells in such a specific and representative way that the off-
spring inherit the modifications even in the slightest degree.
If such a reaction do not occur, Weismann's position is secure;
and though in a system saturated with alcohol, or transferred
to a new climate, the reproductive cells may vary along with
the body, no modification of nerve or muscle can, as such, be
transmitted in inheritance.
The relative scarcity of experimental data, the divergence
of opinion as to the pathological evidence, and the difficulty
of applying our logical or anatomical distinctions to the
intricate facts of nature, make decisive statements impossible,
but it may be said that no clear case of the transmission of an
acquired modification has as yet been forthcoming.
Weismann's position — slightly modified to meet criticism —
must not be held to imply that the germ-cells lead a ** charmed
life," insulated, as it were, from the general life of the body.
THE DEtERMlNATlOK Of SEX. 57
That would indeed be a ** physiological miracle," and it may
be safely said that no one^believes in any such apartness. At
the same time, it may be useful to recall the facts of this
chapter in order to avoid exaggeration of the degree to which
the germ-cells are uninfluenced by modifications in the body.
For in such a case as Yung's tadpoles, influence of nutrition
saturated through the organism and did afiect the reproductive
elements, not indeed to the degree of altering any structural
feature of the species, but yet to the extent of altering the
natural numerical proportions of the sexes. But it must be
clearly understood that this does not really touch the precise
question of the inheritance of acquired characters.
58 THE EVOLUTION OF SEX.
SUMMARY.
1. Nutrition is one of the most important factors in determining sex.
In illustration, note (a) the experiments of Yung, which raised the per-
centage of females from 56 to 92 by good feeding; {d) the case of bees,
where the differences l^tween queen and worker well illustrate the enormous
results of a slight nutritive advantage; also the case of humble-bees, with
three successive broods increasing in nutritive prosperity and in femaleness;
(r) Yon Siebold's experiments with a wasp, which showed most females in
favourable conditions ; (d) Aphides, in prosperity of summer, yield a
succession of parthenogenetic females, in cold and scarcity of autumn males
return; {g) among starved caterpillars of moths and butterflies more males
survive ; (/) Rolph's observations on crustaceans ; (^) experiments on
Rotifers; [A) also the facts noted by Girou, Dilsing, and others, on the
influence of good nourishment of mammalian mothers in favouring female
offspring; {i) the hints of the same results in the human species; (/ ) various
observations in regard to plants favouring the same general conclusion.
2. As to the influence of temperature, favourable conditions again tend
to femaleness of offspring, extremes to males.
3. These factors are now added up — (a) the nutrition, age, &c., of
parents; {d) the condition of the sex elements; (c) the environment of
embryo.
4. The generalisation is thus reached — anabolic conditions favour pre-
ponderance of females, katabolic conditions tend to produce males.
5. But females have been already seen to be relatively more anabolic,
and females relatively more katabiolic. This view of sex is therefore
confirmed.
6. The determination of sex illustrates an outside influence penetrating
to the reproductive cells, but this does not touch the precise question as to
the inheritance of acquired characters.
LITERATURE.
See works mentioned in Chapter III., especially those of Dlising,
Geddes (article Sex, Ency. Brit.), Hensen, and Sutton; also those of
Eimer, Geddes, and Rolph in Chapter II.
Calman, J. T. — The Progress of Research on the Reproduction of the
Rotifera. Nat. Science, XIII., 1898, pp. 43-51 (with bibliography).
DusiNG, C. — As before; also, Die experimentelle PrUfung der Theorie
von der R^ulirung des Geschlecntsverhaltnisses. Jen. Zeitschr. f.
Naturwiss., XIV., Supplement, 1885.
Heykr, F. — Untersuchungen iiber das Verhaltniss des Geschlechtes l^i
einhausigen und zweihausieen Pflanzen, unter Berilcksichtigung des
Geschlechtsverhaltnisses bei den Thieren und den Menschen. Ber.
landwirthschaftl. Inst. Halle, V., 1884, pp. 1-152.
Kerhkrvb, L. B. de. — De Tapparition provoqu?e des m&les chcz les
Daphnies. Mem. Soc. Zool. France, VIII., 1895, pp. 200-211, i fig.
Maupas, E. — Sur le dcterminLsme de la sexuality chez VHydcUitta senia,
C. R. Ac. Sci. Paris, CXIII., 1891, pp. 388-390.
THE DETERMINATION OF SEX. 59
Mbbhan, T. — Relation of Heat to the Sexes of Flowers. Proc Acad.
Nat. Science, Philadelphia (1884), pp. 1 11- 117.
NussBAUM^ M. — Die Entstehung des Geschlechts bei Hydatina setUa.
Arch. Mikr. Anat., XLIX., 1897, pp. 227-308.
Semper, C. — The Natural Conditions of Existence as they affect Animal
Life. Internat. Science Series, London, 1881.
Thomson, J. A. — Synthetic Summary of the Influence of the Environment
upon the Organism. Proc. Roy. Phys. Soc Edin., IX. (1888),
pp. 446-499. (Supplementary to Semper*s work, with bibliography.)
The History and Theory of Heredity. Proc. Roy. Soc. Edin.,
1889, pp. 91-116, with bibliography.
The Science of Life. Londoiy, 1899.
Treat.— Controlling Sex in Butterflies. Amer. Naturalist, VII., 1873.
Wkismann, a.— Die Continuitat des Keimplasmas als Grundlage einer
Theorie dcr Vererbung, Jena, 1885; and numerous other papers,
now translated, in 2 vols. — Essays upon Heredity and Kindred
Biological Problems, authorised translation, edited by £. B. Poulton,
S. Schonland, and A- E. Shipley, 8vo. Oxford, 1899. But especially
*• The Germ-Plasm: a Theory of Heredity," 1893.
WiLCKBNS, M.— Untersuchungen Uber das Geschlechtsverhaltniss und die
Ursachen der Geschlechtsbildung in Haustieren. Biol. Centralblt.,
VL (1886), pp. 503-510; Landwirth. TB., XV., pp. 607-610.
Yung, E.— Contributions h. THistoire de rinfluence des milieux Physiques
sur les Etres Vivants. Arch. Zool. Exper., VII. (1878), pp. 251-282;
(1883) pp. 31-55; Arch. Sd. Phys. Nat., XIV. (1885), pp. 502-522.
De rinfluence de la nature des aliments sur la sexuality. Comptes
Rendus Ac Scl Paris, XCIII., 1881.
De rinfluence des facteurs determinant le sexe. Revue de Morale
Sociale, II., No. 5, 1900, pp. 88-110.
CHAPTER V.
Sexual Organs and Tissues.
IT is the object of this portion of the book to continue the
analysis of sexual characters, but now in a deeper way,
reviewing successively the organs, tissues, and cells concerned
in sexual reproduction. The essential and auxiliary organs of
the two sexes, the frequent combination of these in hermaphro-
dite plants and animals, the sex-cells both male and female,
will be discussed in order. This survey will be for the most
part structural or morphological; the physiological aspects
of sexual union and of fertilisation will be discussed at a later
stage.
§ I. Essential Sexual Organs of Animals. — It is now a well-
established fact that among the ciliated infusorians, which
swarm especially in stagnant waters, a process occurs which
cannot but be described as in part sexual reproduction. Two
individuals, to all appearance alike, become temporarily asso-
ciated, exchange portions of their (micro-) nuclei, and then
separate. This process of fertilisation is essential to the con-
tinued vigour of the species, and will be afterwards described
at length. Such a very simple form of sexual union differs
from what occurs in higher animals in two conspicuous
respects, — {a) the organisms are apparently quite similar in
form and structure ; {b) they are unicellular, and thus there is
no distinction between "body" and reproductive cells. What
is fertilised by the mutual exchange in those infusorians is,
roughly speaking, the entire animal, for the whole is but a
corpuscle of living matter.
Among the Protozoa, however, loose colonies of cells occur,
which bridge the gulf between unicellular and multicellular
animals. In these we. find the first indications of the after-
wards conspicuous difference between "body" and repro-
ductive cells. From these loose colonies, certain of the units
are set adrift, and meeting with others more or less like
64 THE EVOLUTION OF SEX.
themselves, fuse to form a double cell, virtually a fertilised
ovum, from which by continuous division a fresh colony is
then developed. In these transition forms there are thus
reproductive cells of slight distinctness, but as yet obviously
no sexual organs.
When we pass to the sponges, wc find colonies consisting of
myriads of cells, among which there is a considerable division
of labour. An outer layer (or ectoderm) usually consisting of
much subordinated cells, an inner layer (or endoderm) of pre-
dominantly active and we 11- nourished ceils, a middle layer
of heterogeneous constituents, can be distinguished. Every
average infusorian is as good as its neighbours, so far as repro-
duction of new individuals by division is concerned; in the
colonial Protozoa, the units (hat are set adrift are very little
different from their fellows that remain behind; but this ceases
to be true when we pass to colonies where considerable division
of labour has been established. It is certainly true that even
a tiny fragment of sponge, cut off from the larger mass, may,
SEXUAL ORGANS AND TISSUES. 65
if it contain sufficient samples of the body, and if the conditions
be favourable, reproduce a new individual. Cultivators of
bath sponges sometimes take advantage of this fact. But the
sponge starts its new colonies for itself usually in quite a
different way, namely, by the process of sexual reproduction.
Among the cells of the middle stratum of the sponge body
certain well-nourished passive cells appear. These are the ova,
at first very like, but eventually well marked from the other
constituent units of the layer. Besides these there are other
cells, either in the same sponge or in another, which exhibit
very different characters. Instead of growing large and rich in
reserve material like the egg-cells or ova, they divide repeatedly
into clusters of infinitesimal cells, and form in so doing the
male elements or spermatozoa. The male and female cells
meet one another, they form a fertilised ovum; the result is
continued division of the latter till a new sponge is built up.
Here then there are special reproductive cells, quite distinct
from those of the "body"; and here, furthermore, these repro-
ductive cells are markedly contrasted as male and female
elements. As yet, however, there are no sexual organs.
Passing to the next class, the stinging animals or ccelenter-
ates, we find in one of the simplest and most familiar of these,
the common fresh-water hydra, a good illustration of primitive
sexual organs. As in sponges, a cut-off fragment of the body,
if sufficient samples of the different component cells are in-
cluded, is able to reconstitute the whole. But no one body-
cell has of course any such power; this is possible for the
fertilised ovum alone. Now this ovum occurs, not anywhere
within a given layer as in sponges, but always near one spot
on the body. Towards the base of the tube a protuberance
of outer layer cells is developed. This forms a rudimentary
ovary ^ or female organ. It has this peculiarity, not however
unique, that while the organ consists of not a few cells, only
one of these becomes an ovum. A similar protrusion, or more
than one, often at the same time and on the same animal, may
be recognised further up the tube, nearer the tentacles of the
hydra. Smaller than the ovary, each protuberance consists of
numerous small cells, most of which, multiplying by division,
form male elements or spermatozoa. We have here the
simplest possible male organ or testis.
More elaborate organs occur in the other coelenterates,
complicated however by two interesting facts, which will be
5
66 THE EVOLUTION OF SEX.
afterwards discussed, (a) Many of the .coelenterates are well
known to form elaborate colonies, — zoophytes, Portuguese
men-of-war, and the like. In these, division of labour fre-
quently goes further than the setting apart of special organs.
Entire individuals become reproductive "persons" (as they
are technically called), in contrast to the nutritive persons of
the colony, (d) In some of those reproductive individuals,
a curious phenomenon, known as migration of cells, has
been observed by Weismann and others. The reproductive
cells, arising in various parts of the body, have been shown to
migrate in some cases to another part, where they find final
lodgment in more or less definite organs. This occurrence is
intimately associated with " alternation of generations," and will
be afterwards discussed under that heading.
It is far from the purpose of the present work to describe
the details respecting the ovaries and testes, as they occur in
the various classes of animals. It is enough for our purpose to
have emphasised the fact of their gradual differentiation, and
to note that they are almost always developed in association
with the middle layer of the body, and usually occupy a pos-
terior position on the wall of the body-cavity. The details will
be found in any standard work on comparative anatomy, very
conveniently for example in Professor Jeffrey Bell's ** Com-
parative Anatomy and Physiology," London, 1885.
§ 2. Associated Duc/s. — It is only in a few animals, like hydra and its
allies, that the ovaries and testes are external organs, which have simply to
burst to liberate their contents. They are of course usually internal, and
thus arises the necessity of some means of communication with the outside
world. In the simplest cases, the male elements find their way out to the
surrounding medium without any specialised mode of exit. They there
meet, by chance combined with physical attraction at short range, with
the ova, which in the simplest cases again have found their way out in an
equally primitive fashion. Thus in the enigmatical parasitic Mesozoa
(Orthonectids, &c), liberation of the germs may occur by perforation or by
rupture of the excessively simple bodies. In some of the marine worms
{e.g-. Polygordius), the liberation of the ova at least is accompanied by the
fatal rupture of the mother organism, a vivid instance of reproductive
sacrifice. Even in some of the common nereids, the same uneconomical
mode of liberation by rupture appears to occur. The forcible rupture may
be referred to pressure of the relatively large mass of growing cells which
the ovaries often present.
As high up as back-boned animals, the absence of ducts may be traced.
Thus among the sea-squirts or tunicates, the reproductive organs are fre-
quently ductless, and the same thing is true of some fishes. The sex-cells
burst into the body-cavity, and thence find their way to the exterior by
SEXUAL ORGANS AND TISSUES. 67
apertures. In most cases, where ducts are absent, fertilisation of the
ova is external, but this is not necessarily so. In sponges, for instance,
fertilisation is almost always internal. Male elements are washed in by
the water-currents, find their way to the ova, and fertilise them tn situ.
Almost without exception, embryo-sponges, not ova, make their way to the
exterior. In the higher animals, where definite ducts are present, alike for
the inward passage of spermatozoa and the exit of ova or embryos, it
ought further to be noticed that the ovaries can hardly ever be said to be
in direct connection with their ducts. The ova usually burst from the
ovary into the body-cavily, whence they are more or less immediately
caught up by, or forced into the canals, by which they pass outwards.
With the testes it is different, for if diftts be present, they are in direct
connection with the organs.
It is enough to state that in the great majority of cases ducts are
associated with the essential organs. Those of the male serve for the exit
of the spermatozoa, and may be terminally modified as intromittent organs.
Those of the females serve either solely for the emission of unfertilised e^^gs,
or for the reception of spermatozoa, and the subsequent exit of fertilised
ova or growing embryos. In some worm-types, and in all vertebrates,
from amphibians onwards, the reproductive ducts are also in various
degrees associated with excretory functions. For an account of the origin
of the ducts in higher animals, the reader must be referred to the embryo-
logical text-books of Balfour, Hertwig, Haddon, Marshall, and others.
Similarly for such modifications as that of the female duct into oviduct and
uterus, reference must be made to the larger anatomical works of Gegenbaur
and Wiedersheim, or for a briefer account to Paiker's translation and
edition of Wiedersheim*s smaller text-book, and to Professor Jeffrey Bell's
work already mentioned.
§ 3. Yolk-Glands, — As we shall afterwards see, the ovum
is often furnished with a large quantity of nutrient material.
This serves as the food-capital for the growing embryo or young
larva. It is obtained in various ways, — from the vascular fluid,
from the sacrifice of adjacent cells, or from special organs
known as yolk-glands or vitellaria. The yolk-glands, as they
occur for instance in some of the lower worms (turbellarians,
flukes, tapeworms), are of some general interest. They repre-
sent, as Graff has shown, a degenerate portion of the ovary, in
which the cells have become even more highly anabolic than
ova. "The origin of the yolk-gland," Gegenbaur says, "is
probably to be found in the division of labour of a primitively
very large ovary." In more technical language, yolk-glands
are hypertrophied or hyper-anabolic portions of the ovary.
Apart from this nutritive capital, the egg is often equipped
with envelopes or shells of some sort, which may be furnished
by special organs, or by the sacrifice of surrounding cells, or
by the walls of the ducts as the eggs pass out.
§ 4. Organs Auxiliary io Impregnation. — In most animals
68 THE EVOLUTION OF SEX.
in which internal fertilisation of the ova occurs, there are in
both sexes special structures auxiliary to the function of im-
pregnation. Thus the end of the male canal is commonly
modified into an intromittent tube or penis, through which the
male elements flow into the female duct. In the crustaceans
some of the external appendages are often modified, as in the
crayfish, to serve this purpose, and the same is the case with
minute structures on the posterior abdomen of many insects.
Sometimes, as in the snail (f/eli'x\ which may be taken as an
extreme type of reproductive specialisation, separate organs are
present, in which the spermatozoa are compacted into masses
or packets, known as spermatophores. In most cuttle-fishes,
these pass from the male ducts to one of the "arms," which
thus laden is occasionally set free bodily into the mantle-cavity
of the female, where it was of old mistaken for a worm, and
called Heciocotylus, So in some spiders, the palps near the
mouth receive the male elements, and transfer them to the
female. Special storing receptacles and secreting glands are
also very frequently in association with the male ducts, and
there is a long list of curious modifications utilised in the
process of copulation. Thus, male frogs have swollen first
fingers, and gristly fishes have ** claspers," which are modified
parts of the hind limbs, and are inserted into the cloaca of the
female. The common snails eject a limy dart {spiculum amoris\
which appears to be a preliminary excitant to copulation.
So too, in the female sex, the terminations of the duct may
be modified for reception of the male intromittent organ, and
special receptacles may be present for storing the spermatozoa.
Where a single fertilisation occurs, as in the queen bee, previous
to a long-continued egg-laying period, the importance of a
storing organ is obvious. As the female is usually more or less
passive during copulation, the adaptations for this purpose are
less numerous than in the males. It is interesting to notice,
that, among amphibians, where the male often^ takes upon him-
self distinctly maternal duties, one case is known where the
female seems more active than the male during copulation.
§ 5- ^Si'^^yi^g Organs. — Cases where the ova simply pass
out into the water, or on to the land, are of course associated
with the absence of any special organs. In a great many
animals, however, more care is taken, and auxiliary structures
are present. One of the simplest of useful developments is
exhibited by glands, the viscid secretion of which moors the
SEXUAL ORGANS AND TISSUES. 69
ova, and keeps them from being set wholly adrift In insects,
where it is specially important that the eggs should be well con-
cealed, or buried in conveniently nutritive material, hints of
the ancestral abdominal appendages remain as " ovipositors/'
Throughout the series a great variety of structures occur in this
connection.
§ 6. Brooding and Young-Feeding Organs. — From very
lowly animals onwards, structures are present which are utilised
in the protection of the young in their helpless stages. The
reproductive buds of some coelenterates become true nurseries;
in one at least of the marine worms {Spirorbis spirillum), a
tentacle serves as a brood pouch; various adaptations, such as
tents of spines, or cavities in the skin, are utilised in echino-
derms. The young shelter under the hard cuticle, or among
the appendages of crustaceans, and in the gills of some bivalves.
The pockets of not a few fishes, the cavities on the back of the
Surinam toad, the pouches of marsupials, are only a few in-
stances amid a crowd. Sometimes, especially in fishes and
amphibians, — e^,y the sea-horse, with its breast-pouch, and
Rhinoderma darwinii, with its enlarged croaking sacs, — it is
the male which undertakes the brooding office.
When the young are born alive, the internal female ducts
become developed in this connection to form uteri. The ovary
appears to serve as a womb in the genus Girardinus among
fishes, but it is usually the median portion of the female duct
which has this function. In placental mammals, where the
young are born at an advanced stage, and where the maternal
sacrifice is at its maximum, the uterine adaptations become
more important and complex. The organs of lactation will be
afterwards discussed.
In illustration of the strange inter-relations between difierent
forms, we may refer to the fresh-water mussels. The larvae or
Glochidia are sheltered and nourished in the outer gill-plates
of the female, and are liberated when fishes come near. To
the skin of these the larvae fix themselves, become temporarily
parasitic, and undergo a metamorphosis, after which they fall off.
Without the presence of fishes the life-history cannot be com-
pleted. On the other hand, the young stages of the fresh-
water fish known as the bitterling {Rhodeus amarus) find
temporary shelter in the gills of the mussels.
70 THE EVOLUTION OF SEX.
SUMMARY.
1. The gradual differentiation of essential sexual organs in animals, —
isolated cells, aggregated tissues, definite organs.
2. Associated male and female ducts for the liberation of male-elements,
fertilisation, exit of ova, or birth of embryos.
3. Yolk-glands, &c., for nourishment and equipment of the ova.
Vitellaria have been interpreted as d^enerate ovaries.
4. Illustrations of organs auxiliary to impregnation. In the male, —
penis, storing sacs, spermatophore-making organs, " clasj>ers." Curiosities,
such as the hectocotylus of cuttle-fishes, and the Cupid's dart of snails.
Adaptations in the female are less frequent, but storing receptacles for the
male-elements are common.
5. Egg-laying organs : — frequency of ovipositors.
6. Brood-pouches and the like are widely present in most classes of
animals.
LITERATURE.
Balfour, F. M. — A Treatise on Comparative Embryology. 2 vols.
London, 1 881.
Bell, F. Jeffrey. — Comparative Anatomy and Physiology. London,
1885.
Claus, C. — Elementary Text-Book of Zoology, trans, by A. Sedgwick.
2 vols. London, 1885.
Geddes, Y.—Op, a'L
Gegrnbaur, C. — Elements of Comparative Anatomy, trans, by Prof.
Jeffrey BelL London, 1878.
Haddon, a. C. — An Introduction to the Study of Embryology. London,
1887.
Hensen, V. — Op. at,
IIertwig, O. — Lehrbuch dcr Entwicklungsgeschichte des Menschen und
der Wirbelthiere. Jena, 1888.
Hatchett Jackson's (W.) Edition of Rolleston's Forms of Animal Life.
Oxford, 1888.
Huxley, T. H. — Anatomy of Vertebrate and Invertebrate Animals.
London, 1871 and 1877.
Sachs, J. — Ttxt-Book of Botany, edited by Prof. Vines. Second edition.
Oxford, 1882. And similar works.
Lectures on the Physiology of Plants, trans, by Prof. Marshall
Ward. Cambridge, 1887.
Vines, S. H. — Vegetable Reproduction (Ency. Brit.). Lectures on the
Physiology of Plants. Cambridge, 1886.
Wiedersheim, R. — Elements of the Comparative Anatomy of Verte-
brates, trans, by Prof. \V. N. Parker. London, 1897. Also un-
abridged work.
CHAPTER VI.
HERMAPHRODITISM.
§ I. When an organism combines within itself the production
of both male and female elements, it is said to be bisexual or
hermaphrodite. This is the case with many of the lower
animals, — such, for instance, as earthworms and snails. It is
not desirable to extend the term, as is sometimes done, to
cases like ciliated infusorians, where sex itself is only incipient
In flowering plants, the stamens and carpels, which produce
microspores and macrospores respectively, are often, though
inaccurately, called the male and female organs, and when both
are present in the same flower the term hermaphrodite is often
used. But the flowering plant is a sporophyte with only a
vestige of the sexual generation left, and the term hermaphro-
dite should be kept for cases like the prothallus of a fern — a
sexual generation with male and female organs on the same
small expansion. While the general definition of hermaphro-
ditism, as the union of the two sexes in one organism, is plain
enough, the union is exhibited in a great variety of ways and
degrees. Of these it is necessary flrst to take account
§ 2. Embryonic Hermaphroditism, — Some animals are
hermaphrodite in their young stages, but unisexual in adult
life. Allusion has already been made to the case of tadpoles,
where the potential bisexuality occasionally lingers into adult
life. According to some, most higher animals pass through a
stage of embryonic hermaphroditism, but decisive proof of this
is wanting.
The research of Laulanie may now be referred to at greater length. As
the result of observations on the development of the reproductive organs in
the higher vertebrates, and especially in birds, he seeks to establish a strict
parallelism between the individual, and what he believes to have been the
racial history. In the chick, he distinguishes three main stages in the
development — (i) germiparity, (2) hermaphroditism, (3) differentiated
unisexualily. These he regards as recapitulating the great steps of the
historic evolution, (i.) For the first period of "germiparity," — from the
7^ tHE EVOLUTION OF SE3t.
to the sixth day, — the designation, sexua] neutrality, or indifference,
mropriate, since the "cortical ovules" of the germinal epithelium
fourth
is inappropriate,
have from the first the precise morphological significance of female ele-
ments or ova. In the female, they proceed by multiplication to form the
ovary; in the male, they degenerate. (2.) The period of hermaphroditism
begins with the seventh day. In the male, the male ovules, from which
the sperms are afterwards developed, appear in the central tissue ; but at
the same time cortical or female ovules may be seen persisting. Similarly,
in the developing ovary of the female, the central or medullary portion,
strictly separated by a partition of connective tissue from the egg-forming
layer, contains a large number of medullary or male ovules. (3.) This
hermaphroditism is of short duration. The cortical or female ovules
disappear from the testes by the eighth or ninth day; and the medullary
or male ovules have by the tenth day disappeared from the ovary. In
regard to mammals, Laulanie affirms, allowing some peculiarities, that the
same three stages of germiparity, hermaphroditism, and unisexuality occur.
Ploss has already l)een referred to as another investigator who main-
tains the existence of embryonic hermaphroditism. Such also is the view
held by Professor Sutton, who concludes that both sets of organs are
equally developed up to a definite period, and emphasises the consequent
necessity for the hypertrophy of one sexual rudiment over the other.
§ 3. Casual or Abnormal Hermaphroditism. — In many species which
are normally unisexual, a casual hermaphrodite form occasionally presents
itself. The embryonic equilibrium or bisexuality — one of the two must in
a variable degree exist — is retained as an abnormality into adult life
Kven as far up in the organic series as birds and mammals, such casual and
yet true hermaphrodites occur. In most cases at least the result is sterility.
Among amphibians, which abound in reproductive peculiarities, herma-
phroditism exceptionally occurs, and in some species of toad it seems to be
constant. The common frog, so much dissected in our laboratories, has
supplied several good illustrations. Thus Marshall notes that the testes
may be associated with genuine ova, or an ovary may occur on one side,
and a testis with an anterior ovarian portion upon the other. Bourne gives
a case of a frog with the ovary well developed on the right side, and
opposite this an ovary anteriorly replaced by testis. One of the toads
{Pelobaits fuscus') seems to be frequently hermaphrodite, the male being
furnished with a rudimentary ovary in front of the testes. A similar
hermaphroditism is not at all infrequent in cod, herring, mackerel, and
many other fishes. Sometimes a fish is male on one side, female on the
other, or male anteriorly and female posteriorly. Sir J. Y. Simpson, in a
learned article on the subject, has distinguished cases of true hermaphro-
ditism, according to the position of the organs, into lateral, transverse, and
vertical or double. Among invertebrates the same has been occasionally
observed, especially among butterflies where striking differences in the
colouring of the wings on the two sides have in some cases been found
to correspond to an internal co-existence of ovary and testis. The same
has been observed in a lobster, and is probably commoner than the
recorded cases warrant one in asserting. As low down as coelenterates,
casual hermaphroditism may occur, as F. E. Schulze showed in one of the
medusoids.
§ 4. Pariial Hermaphroditism. — An organism may be said to be truly
hermaphrodite when both male and female organs are present, or when.
HERMAPHRODITISM. 7 3
without there being separate organs, both male and female elements are
produced. It is then both anatomically and physiologically hermaphro-
dite, and of this, as we shall see, there are abundant illustrations among
lower animals. Snail, earthworm, and leech are examples of this herma-
phroditism, in varying degrees of intimacy.
But, as we have just noticed, a species normally unisexual may occasion-
ally exhibit hermaphrodite individuals. In these only one of the double
essential organs may be functional, or l)oth may be sterile. Whether
physiologically or not, such animals are anatomically hermaphrodite. Both
kinds of essential organs are at least present.
To those must now be added a further series of cases to which the term
partial hermaphroditism seems most applicable. Only one kind of sexual
organ, ovary or testis, is developed; but while one sex preponderates, there
are more or less emphatic hints of the other. As the reproductive organs
are to be regarded as the most important, but not by any means the sole
expression of the fundamental sex-differences, it is impossible to separate
partial hermaphroditism by any hard and fast line from the above, and
from the next set of cases (paragraphs 3 and 5). Almost all cases of partial
hermaphroditism occur as exceptions, though a few are constant.
In the higher animals, partial hermaphroditism is usually expressed in
the nature of the reproductive ducts. In this connection the structural
resemblance of the male and female organs must be once more emphasised.
Even the Greeks had their vague and fanciful theories of what we now call
the homology of the reproductive organs and ducts in the two sexes.
Through the labours of the anatomists of Cuvier*s school, such as his
fellow-worker Geoffroy St Hilaire, and yet more through more recent
embryological discoveries, there is now both clearness and certainty as to
the main facts. The reproductive organs proper, the ducts, and the
external parts, are developed upon the same plan in male and female.
Thus, except in the lowest vertebrates, what serves as an oviduct in the
female, is equally present in the embryo male, and persists in the adult as
a more or less functionless rudiment. In the same way, what serves as
the duct for the sperms {vas deferens) in the male is equally present in the
embryo female, and persists in the adult as a rudiment, or is diverted to
some other purpose. This is a perfectly normal occurrence, dependent
upon the embryological history of the ducts in question. It is necessary,
however, to realise both the primitive resemblance and the fundamental
unity of the two sets of organs, in order to understand how partial herma-
phroditism is so frequent, and also to distinguish it from '* spurious
Hermaphroditism,'' where a merely superficial abnormality or even injury
of the ducts in one sex produces a resemblance to those of the other.
We have already mentioned that in the case of twin calves, two females
may occur, and both are then normal ; or two normal twin calves may he
born of opposite sexes ; but, in the third place, if both be males, one of
these very generally exhibits the peculiar phenomena of what is called a
" free-martm." In the commonest form of this, partial hermaphroditism
is well illustrated. The essential organs are male, but there is a rudi-
mentary uterus and vagina, and the external organs are further those of a
female.
It is necessary to note that a simulation of even this partial hermaphro-
ditism may result from malformation or rudimentary development of the
external organs. On this subject we may quote an acknowledged autho-
74 THE EVOLUTION OF SEX.
rity, alike in anatomical and embryological matters. '* From the fact,"
Prof. O. Her twig remarks, ** that the external sexual organs are originally
of uniform structure in the two sexes, wc can understand the fact that, in
a disturbance of the normal development, forms arise in which it is ex-
tremely difficult to decide whether we have to deal with male or female
external organs. These cases, in earlier times, were falsely interpreted as
hermaphroditism. They may have a double origin. Either they are
referable to the fact that in the female sex the development may proceed v
along the same path as in the male, or to this, that in the male the normal
development may come at an early age to a standstill, and lead to the
formation of structures which resemble the female parts." In the first
case, he goes on to say, there may be a simulation of a penis, and the
ovaries may even be shifted so as to produce an appearance like that of
the testes within their scrotal sac. In the second case, the processes of
coalescence which give rise to the penis may not occur, only a rudimentary
organ is formed, and there may even be an inhibition of the usual descent
of the testes into their sacs.
Of this superficial hermaphroditism, really not hermaphroditism at all,
there are numerous cases among mammals. But there remain a large
number of recorded instances, where the anatomy of the ducts was pre-
dominantly that of the sex opposite to that indicated by the essential
organs, and where the combination of the two sexes was also expressed in
external configuration and even in habit. Amphibians again furnish some .
interesting examples. Attached to the anterior end of the testis in various '
species of toad {Bu/o), there is an organ known as *' Bidder's," which has
contents like young ova. These do not, however, get past the early stages,
and the organ is quite different from the more than rudimentary ovary
which occurs constantly in the males of Bu/o cinereus and some other
species. The two may in fact occur together. In the common frog,
dissectors have also recorded several cases of hermaphroditism expressed in
the ducts. Lastly, it is perhaps not going too far to include here some
reference to the curious ** fatty bodies which occur in all amphibians at
the apex of the reproductive organs in both sexes. These appear to
nourish the ovary and testis, especially during hybernation, and may
perhaps be associated with similar lymphoid structures in fishes and
reptiles. Prof. Milnes Marshall demonstrated that the fatty bodies result
from the degeneration of the anterior part of the reproductive organ while
still in an indifferent state.
Leaving the ducts out of account, we may arrange the
important phenomena of hermaphroditism in amphidians in a
series as follows : —
[a) Embryonic hermaphroditism, demonstrated as of normal occurrence
in frog tadpoles.
{b) Casual hermaphroditism, demonstrated in frogs, e.g. in the occur- /
rence of distinct ova in the seminiferous tubules of Rana viridis as reported r
by F. Friedmann (Arch. Mikr. Anat., Hi. (1898), pp. 248-262, i pi.
i expressed in Bidder's organ in male toads;
(<:) Partial hermaphroditism, -| (aUo expressed in various stales of the
\ ducts).
[d\ Normal adult hermaphroditism, in some species oiBufo,
HERMAPHRODITISM. 75
The list need not be further followed; it is enough to note
the very wide occurrence of partial hermaphroditism. In many
cases, moreover, we find what may be called superficial her-
maphroditism, expressing itself in the external characters.
Forms occur in which the minor peculiarities of the two sexes —
colouring, decorations, weapons, and the like — appear blended
together, or in which the secondary sexual characters are at
variance with the internal organs. In most cases, one is safe
in saying that there is no true internal hermaphroditism in any
degree. Arrest of maturity or puberty, cessation of the repro-
ductive functions, removal or disease of the essential organs,
and the like, may alter the secondary sexual characters from
female towards male, or, less frequently, vice versd, A female
deer may develop a horn, or a hen a spur, and in such cases
the ovaries are generally found to be diseased. The prettiest
cases of superficial hermaphroditism occur among insects,
especially among moths and butterflies, where it often happens
that the wings on one side are those of the male, on the other
those of the female. Only the external features have been
observed in most cases; but it has been shown by dissection
that such superficial blending may exist along with internal
unisexuality, or, in a few cases, with genuine internal her-
maphroditism. A beautiful case of intimate blending of
superficial sex characters was lately shown to us by Mr W. de
V. Kane of Kingstown. A specimen of butterfly {Euchlot
cuphenoides) showed the anterior half of the fore wings and
part of the hind wings with the characteristic white ground of
the female, while in the posterior half of the fore wings and on
most of the hind wings the characteristic sulphur of the male
prevailed. In other minor ways, the characteristics of the two
sexes, which are well marked, were intimately blended. In
all such cases we may suppose that the imperfection of the
normal unisexual dififerentiation removes the usual limits to
the appearance of this or that secondary sexual character.
§ 5. Normal Adult Hermaphrodilism, — This is rare among
the higher animals, but common among the lower. On the
threshold of the vertebrate series, we find it indeed constant
among the Tunicata; but above these it is very rare. The
hag {Myxine) was shown by Cunningham, and afterwards by
Nansen, to be a protandrous hermaphrodite, but this conclusion
is contested by Bashford Dean (" Festschrift Kupfier," 1899).
"A testis is constantly found imbedded in the wall of the
76 THE EVOLUTION OF SEX.
ovary in Chrysophrys and Serranus^ and the last-named fish
is said to be self-impregnating." In some species of male toad
{e.g.^ Bufo cinereus) a somewhat rudimentary ovary is always
present in front of the testes. All other cases among verte-
brates are either casual (par. 3) or partial (par. 4). Among
invertebrates, true hermaphroditism is frequent.
(1.) Sponges. — As already mentioned, the sex-cells of sponges arise
among the components of the middle layer {nusoglcecC) of the body. It is
at least possible that in any sponge they may develop either into ova or
into sperms, or into both, withm the same organism, according to nutritive
and other conditions. The facts, however, are these. Many sponges are
only known in a unisexual state, while others are genuinely hermaphrodite.
But among the latter it is not uncommon to find (^.^., in Sycandra
rapkanus) that the production of one set of elements preponderates over
the other, and thus we have hermaphrodites with a distinctly male or
female bias. In other words, they are verging towards uniscxuality. It
does happen in fact {cg,^ in Oscarella iebularis) that a species normally
hermaphrodite may exhibit unisexual forms.
(2. ) CaUnieraies. — The members of this class are higher, in having the
production of the sex-cells more restricted, to definite regions, tissues,
organs, or even ** persons.** The highly active Ctenophores, like Berbe^
are all hermaphrodite, and that very closely. On one side of the meri-
dional branches of the alimentary canal ova arise, on the other side sperma-
tozoa. Among sea-anemones and corals the hermaphrodite condition
appears in a number of cases, but is sometimes obscured by the fact that
the two kinds of elements are produced at different times, corresponding to
different physiological rhythms in the life of the organism. The genus
Corallium (the red coral of commerce) is peculiarly instructive. The whole
colony may be unisexual, or one branch of the colony, or only certain
individuals on a branch, while genuine hermaphroditism of individual
polyps also occurs. Among hydrozoa (zoophytes, swimming-bells, jelly-
fish), hermaphroditism is a rare exception. The common hydra, which is
a somewhat degenerate type, is hermaphrodite, though at the same time
individuals may be found with only ovary or only testes. Eleutheria is
also hermaphrodite, and abortive ova occur in the male of Gonothyrea
loveni. Sometimes a colony is hermaphrodite {Dicoryne)^ but the stems
and individuals unisexual. Sometimes a stem is hermaphrodite, but the
individuals unisexual (certain sertularians). Among jelly-fishes tlie genus
Ckrysaora is known to be hermaphrodite.
(3.) ** IVorms," — The condition of the sexual organs varies enormously
among the diverse types lumped together under the title of "worms" or
*' Vermes." In the lowly turbellarians, all the genera are hermaphrodite
except two, but, as in many other cases, the organs do not reach maturity
at the same time, the male preceding. In the related trematodes or flukes,
hermaphroditism again obtains, with one exception, or perhaps two. The
certain exception is the curious parasite Bilharziay where the male carries
the female about with him in a ^'gynsecophoric canal," formed of folds of
skin. In the adjacent class of cestodes or tapeworms, all the members are
hermaphrodite, with one alleged exception. The utility of the herma-
phrodite state, if the eggs of these parasitic animals are to be fertilised and
HERMAPHRODtTISH. 77
the species miinlained, can hiidly be Joubted. It U imporlant to notice
too, that seir-fertilisaiion — that is, union of the e^ and sperms of the
same organism — has been proved to occur in several tremntodes, and seems
to be almost universal in cestodes. This may be parity a cause and pailly
a consequence of the degeneracy of these parasites, for fiequenl as heiina-
phrodilism is among plants and animals, self- fertilisation is extremely raie.
Hermaphroditism is rare among the free-living nemerteans, bul con-
stant in the semi- parasitic leeches. An exception to sepaialeness of the
sexes among threadworms or nematodes is found in Ihe cuiious genus
Angiostomutn. Here, in an organism which is anatomically a female, the
reproductive organ begins its activity by pro-
ducing spermatoioa, which fertilise the subse-
quenl ov>, The animal is thus physiulogically
hermaphrodite, and at the same time self-
impregnating. Approaching the higher anne-
lid worms, we find the primitive PrQlodriluS
hermaphrodite; the earthworms are constantly
so, but all their marine relatives have the sexes
separate. The genus Sagilta, which stands b^
itself, is heimaphrodile ; the same condition is
known as a ranty among the ancient brachio-
pods {Lingttla), but is frequent among the
colonial Potyzoa. Many, at least, of the My-
loslomala — aberrant parasites of Ctinoids — are
hermaphrodite.
(4.) ficAimw'ijrOTii.— Almost all the merti-
bers of this class have separate sexes. Among
the few exceptions are the species of Synapta
(a illvergenl Hololhuroid), a sand-slar {Aoipki-
vra squanuUa), and a starfish {AUerina gib-
be!a\. The last is particularly interesting. At
KoscolT, the individuals are males for one or
two years, and then become females; at Ban-
yuls, the individuals are males for at least two
or three years, but eventually become females ;
at Naples some are wholly male, olheis Biihuria, . oaraMlic iremalmfc,
wholly female, others hermaphrodite impani- {„ ^hich i^male cnrria the
ally, others transitional. (See L. Cu^not, ftmaleLnaipMial Wdoltkin
Zool. Anieiger, XXL, .B98, pp. ^n-m. 3 ^.-JlSt« KHSIS!™'
(5.) ATihrepeds.—kxnrm^ crustaceans, hermaphroditism is a rare ex-
ception, though it occurs in Ihe majority of Ihe fixed quiescent acorn'Shells
and barnacles (Cirri petliat. There il is associated with the presence of
small males, which Darwin called " complemenial." The Cymothoidic
(Isopods) show a curious condition somewhat like (hat of Angiaslomum
above noticed. The sexual organ of the young animal is thale, of the
old, female In function. In such cases, one must remember the antithesis
between the body proper and (he reproductive cells. In youth the demands
of the body during growth are greater; there is no anabolic surplus to
spare, all goes to increase the body. When mature siie is reached, and
growth and activities are lessened, (here is t""'- lii'-iii'n™! ^f anntmlir
preponderance in the reproductive, as opposed
78 THE EVOLUTION OF SEX,
Myriopods and insects have always separate sexes, exclnding of course
abnormal hermaphroditism among the latter.
(6.) Molluscs, — Most bivalves are of separate sexes, but exceptions
often occur — e.g.y in common species of oyster, cockle, clam, &c. In the
case of the oyster, the familiar species {Oslrea edulis) is hermaphrodite,
and a neighbouring species apparently unisexual. In both cases the organs
are the same, but in O. edulis the same intimate recesses of the reproduc-
tive organ produce at one time ova, at another time sperms.
The snails, or gasteropods, are divided into two great groups, according
to the twisting of their nerves. The one group (Streptoneura) have the
sexes separate; the members of the other series (Eulhyneura) are her-
maphrodite.
The sea-butterflies, or pteropods, are hermaphrodite, but the elephant's
tooth shells (Scaphopods) are unisexual So in cuttle-fishes (Cephalo-
pods), the sexes are separate.
In the limpet (Patella) hermaphroditism occurs as a casual variation,
3 out of 250. The low-level limpets, which are probably better fed, show
no preponderance of females, but it may be noted that the female repro-
ductive organ of the limpet is not larger than the male organ, and does not
therefore make special demands on nutrition. (See J. F. Gemmill^ Anat.
Anzeiger, XII., 1896, pp. 393, 394.)
§ 6. Degrees of Normal Hermaphroditism, — From what has
been already said, it is evident that hermaphroditism may be
more or less intimate. Thus the red coral is sometimes female
as regards one branch, and male as regards another ; a leech
has the ovaries far forward, and independent of the long row
of testes; in a tunicate the testes and ovary may form one
mass, the male cells spreading over the surface of the ovary.
In the same way, the organ of a scallop, which exhibits more
or less distinct male and female portions, is in a state of less
intimate anatomical hermaphroditism than the oyster, where
the same caeca of the same organ fulfil both functions at
different times.
This last caution must be kept in view, for there is through-
out, in varying degrees, a tendency to periodicity in the
production of male and female elements. Such a want of
** time-keeping" between the sexes is called dichogamy, and
is one of the conditions which render self-fertilisation rarely
possible. The male function has in the majority of cases the
precedence. Similarly in flowering plants, although it is not
quite accurate to call the stamens the male organs, or the
carpels the female organs, it may be said that '* protandrous
dichogamy" (stamens taking the lead) is very much commoner
than "protogynous dichogamy," where the carpels are first
matured. This agrees with the curious cases of Angiostomum
HERMAPHRODITISM. 79
and Cyniothoidas already mentioned, where the organ was first
male and then female, and indeed with at least most cases
among closely hermaphrodite animals. Where the male organs
are situated in one part of the body, and the female in another,
there is less reason against the production of sperms going on
at the same time as the production of ova. In terms of our
general thesis, protogyny corresponds, like unisexual female-
ness, to a relative predominance of anabolism in the life-ratio,
and protandry with the reverse.
The common snail (Heiix) is not only easily dissected, but
in the complexity of its arrangements is full of interest. Here,
not only are ova and sperms produced within the compass of
one small organ, but each little corner of the organ shows
female cells forming on the walls and male cells in the centre.
It has been suggested by Platner that the outer cells are the
better nourished; they therefore naturally become developed
into anabolic ova. In the large slug, Limax maximus^ Babor
found a succession of sexual states, — female, hermaphrodite,
male, hermaphrodite, female; and suggests that the same
alternation may be observed elsewhere. (Verb. Zool.-bot.
Ges. Wien, xlviii., 1898, pp. 1 51-153.)
§ 7. Self-Fertilisation, — We have noted above, that though
male and female organs be present in the same organism, they
tend to become mature at different times, and that the more
the closer the seats of formation of the two kinds of elements.
It is equally necessary to emphasise that, though both male
and female elements may be produced in the same plant or
animal, it is probably exceptional for the ovule to be penetrated
by a pollen cell from the same flower, and it is certainly rare
for an animal to fertilise its own ova.
It is believed by breeders of higher animals that ** close-
breeding " beyond a certain point is dangerous to the welfare of
the stock. The offspring tend to be abnormal or unhealthy.
In view of this, the rarity of self-fertilisation among herma-
phrodites has been explained in terms of the disadvantage of
the process. In reality, however, this is putting the cart before
the horse. In hermaphrodites, we take it that the two kinds of
sexual elements mature and are liberated at different times, not
because of any reaction of the disadvantageousness of self-
fertilisation on the health of the species, but simply because the
simultaneous co-existence of opposite physiological processes is
in varying degrees prohibited. More technically, dichogamy is
8o THE EVOLUTION OF SEX,
not a secondary result of ihe disadvantage of self-fertilisation
nor of the advantage of cross-fertilisation, but increasing dicho-
gamy is the primary condition of cross-fertilisation.
Self-fertilisation does, however, occur as an exception among
animals, — thus in all probability in the interesting fish Ser-
ranus; certainly in many parasitic flukes or trematodes; com-
monly, if not almost always, in tape-worms or cestodes; also
in the curious thread-worm Angiostomum, and probably in
ctenophores, and in some other cases. In regard to some
cases, e.g,^ among hermaphrodite bivalves (where the sperms
are usually wafted in with the water), it is impossible as yet to
say whether self-impregnation does or does not occur.
Arguing from the bad effects of close breeding among
higher animals, Darwin and others have called attention to the
numerous contrivances among plants which are said to render
self-pollination impossible. In some cases the pollen of a
given flower is quite inoperative on the ovule of the same
flower, or has the result of producing weakly oflspring. There
are also many mechanical devices, as the result of which
it is difficult or impossible for the pollen of the stamens
to reach the stigmas of the flower, or even to be dusted upon
them by the unconscious agency of the intruding insects.
Moreover, as among animals, so among plants, it is common
for the stamens to become mature before the carpels are ready,
or, in rarer cases, for the reverse to occur.
There is no doubt that cross-fertilisation very generally
occurs, and it is physiologically probable that this is a con-
siderable advantage, though probably less among plants than
among animals. But there is an increasing impression that
both the occurrence of cross-fertilisation, and the necessity of
it among higher plants, have been exaggerated. One of the
most thoughtful and observant of American botanists, Mr T.
Meehan, has raised a vigorous protest against the prevalent
view. In the Yucca^ or Adam's needle, which is regarded as
cross-fertilised by insects, he showed by experiment that there
was in each flower " no abhorrence of its own pollen." " Even
when fertilised at all by insects, I am sure the fertilisation is
from the pollen of the same flower."
Then as to mechanical contrivances, he says, " we are told
that iris, campanula, dandelion, ox-eye daisy, the garden pea,
lobelia, clover, and many others, are so arranged that they
cannot fertilise themselves without insect aid. I have enclosed
HERMAPHRODITISM. gj
flowers or all those named in fine gauze bags, and they produced
seeds just as well as those exposed."
We cannot here enter into a full statement of Meehan's
careful observations, but his three main propositions well
deserve statement and due consideration:—
1. Cross-fertilisation by insect agency does not exist nearly
to the extent claimed for it.
2. Where it does exist, there is no evidence that it is of any
material benefit to the race, but to the contrary.
3. Difficulties in self-fertilisation result from physiological
disturbances that have no relation to the general welfare of
plants as species.
82 THE EVOLUTION OF SEX.
§ 8. Compkmental Males. — When Mr Darwin was inves-
tigating barnacles and acorn shells, in preparation for his
monograph on the group, he discovered the remarkable fact
that some of the hermaphrodite individuals carried minute
males concealed under their shells. These he regarded as
advantageous accessory forms, ensuring cross-fertilisation in the
hermaphrodites which harbour them. The great majority of
the cirripedes are hermaphrodite; but among the barnacles
proper, — the stalked forms, which are nearer the ancestral type,
— separate sexes sometimes occur. On the females of a few
of these, pigmy males, like those found upon hermaphrodites,
also occur. These pigmy males, whether on females or herma-
phrodites, are not only dwarfish, but are very often degenerate,
sometimes wanting (according to Darwin) both alimentary
canal and thoracic legs. Some of them, in fact, are little more
than parasitic testes.
The various steps in the evolution may be hypothetically
sketched : —
(d) The original state of affairs was probably the ordinary crustacean
condition of separate sexes, [b) A second stage may have been repre-
sented by a diminution in the size of the males, as in some of the '* water-
fleas " or copepods, while the females became more and more sluggish, and
settled down, {c) In the genera Alcippe and Cryptophialus^ in the species
Ibla cummingii and Scalpellum omaium, there are true females, with
attached pigmy males, often several, leading a shabby existence as parasites.
{d) In other species of Scalpellum and Ibia the same pigmy males occur, but
attached, as we have noted, to hermaphrodites, which in these forms have
replaced the true females, {e) Lastly, in many genera, like Pollicipesy
only hermaphrodites occur.
In a description of the complementary male of Scalpellum vul^are, A.
Gruvel notes ("Archives de Biologic," xvi., 1899, pp. 27-47, i pi.) that it
in many respects follows the hermaphrodite form, but is greatly simplified,
e.g,y in the absence of alimentary canal and of specialised vascular and
respiratory organs. He suggests that as the spermatozoa of the hermaphro-
dite form ripen before the eggs, some of the belated eggs may be fertilised
by the spermatozoa of the complementary male which is later in attaining
maturity. He supposes further that these belated eggs give rise to the
next brood of complementary males. But this, as is the case with so many
of these speculations, awaits experimental verification.
What Darwin did for the cirripedes, Graff and others have done for
another very curious set of animals, the Myzostomata. These are degene-
rate chaetopods or bristle-footed worms, which occur as outside parasites on
sea-lilies (crinoids), on the arms of which they make curious galls. There
is unfortunately lack of agreement among observers, and the life-history of
these curious forms requires further study. We can only indicate two
different sets of results.
According to Beard, " the various kinds of parasitism presented by the
HERMAPHRODITISM. 83
numerous species of Myzostoma have led in some cases to the preservation
of males, in others to their extinction, in yet others to their conversion into
hermaphrodites." He distinguishes —
1. Purely dioecious forms with small males, e.g. M. fulvinar.
2. Hermaphrodite forms and true males, which remain as dwarf
" complemental males" on the back of the hermaphrodites, e.g.
M. glabrum,
3. Hermaphrodite forms and males, which, retaining their position
on the back of the others, afterwards become females, e.g.
M. alatum.
4. Hermaphrodite forms, in which the males have lost their dorsal
position, and have either become extinct or converted into pro-
tandric hermaphrodites, e.g. M. cirriferttm.
According to Wheeler, Myzostoma glabrum is from the first herma-
phrodite and not dimorphic, but a functional male phase is succeeded by a
functional hermaphrodite phase, and that again by a functional female phase
during which the testes disappear.
" The cysticolous and endoparasitic species of the genus tend towards
a condition in which the functional male and female phases overlap but
little, thus exhibiting only a brief hermaphrodite phase {^M. eremiia)^ or
these phases no longer overlap and thus present two well-marked periods
of sexual maturity, one male and the other female {M. pulvinar).^
§ 9. Conditions of Hermaphroditism. — A review of the
occurrence of normal hermaphroditism suggests few general
conclusions. Glaus points out that hermaphroditism finds
most abundant expression in sluggish and fixed animals.
Flukes, tapeworms, leeches, even earthworms and land-snails,
may illustrate the sluggard hermaphrodites; among sponges,
sea-anemones, corals, Polyzoa, bivalves, &c., we find frequent
illustration of the association of fixedness and hermaphroditism.
Most of the tunicates are also fixed, and all are hermaphrodite.
But the pelagic tunicates are also hermaphrodite, and so are
the very active Ctenophora. Glaus notes further that in flukes
and tapeworms hermaphroditism is associated with an isolated
habit of life. But there is often anything but isolation, for
flukes may occur near one another in great numbers; and as
many as ninety tapeworms {Bothriocephalus) have been known
to occur at one time in a single host
Simon has gone further, in insisting on the physiological
connection between quiescent and parasitic habit and the
hermaphrodite condition. In flukes and tapeworms, leeches,
Myzostomata, and some cirripedes, we find the association of
hermaphroditism with a more or less intimate parasitic habit.
But what Simon points out is, that organisms on which great
demands are made, especially in the way of muscular exertion,
84 THE EVOLUTION OF SEX*
cannot aflbrd to be hermaphrodite; while a plethora of nutrition,
as in parasitism, tends to make the persistence of the double
state possible. He gives numerous illustrations in support of
this view. Others are content to interpret the hermaphroditism
in all these cases as an adaptation to ensure fertilisation, for
the possibilities of pairing between separate sexes are certainly
lessened if the animals are sluggish, sedentary, or parasitic.
§ TO. Origin of Hermaphroditism, — (i) One view of the
matter is that hermaphroditism was the primitive state among
multicellular animals, at least after the differentiation of sex-
elements had been accomplished. In alternating rhythms,
eggs and sperms were produced. The organism was alternately
male and female. Of this primitive hermaphroditism, there
may be more or less of a recapitulation in the life-history of the
organism. Gegenbaur states the common opinion in the
following cautious and terse words: — "The hermaphrodite
stage is the lower, and the condition of distinct sexes has
been derived from it." Unisexual "differentiation, by the
reduction of one kind of sexual apparatus, takes place at very
different stages in the development of the organism, and often
when the sexual organs have attained a very high degree of
differentiation." The first structural stage in the separation
would probably be the restriction of areas, in which the forma-
tion of two kinds of cells still went on at different times in one
organism. In different individuals the opposite tendencies we
have already spoken of more and more predominated, till
unisexuality evolved out of hermaphroditism.
That environmental conditions are effective in changing
the hermaphrodite into the unisexual state is suggested by
many experiments. And it has been shown in regard to some
flowering plants, eg, butcher's broom {Ruscus aoileatus\ that
the monoecious or dioecious condition may be evoked by alter-
ing the nutritive conditions.
(2) Quite different is the view which regards hermaphro-
ditism as a secondary condition, derived from primitive uni-
sexuality. Thus Pelseneer maintains that the "study of
Mollusca, Myzostomidae, Crustacea, and Pisces shows that
in these groups the separation of the sexes preceded herma-
phroditism; various cases in other groups tend to show that
this is true universally; and the same conclusion applies to
plants. In Mollusca, Crustacea, and Pisces, at least, herma-
phroditism is grafted upon the female sex.''
HERMAPHRODITISM. 85
SUMMARY.
1. Hermaphroditism is the union of the two sexual functions in one
organism. Tnis occurs, however, in varying degrees.
2. Embryonic hermaphroditism is probably a general fact with even
unisexual animals. It is certain in some cases.
3. Casual or abnormal hermaphroditism is not infrequent.
4. Partial hermaphroditism (not involving the essential organs) is
exceedingly common.
5. Normal adult hermaphroditism ; review of its occurrence.
6. True hermaphroditism occurs in many degrees of intimacy.
7. Self- fertilisation is a rare exception among animals; commoner in
plants.
8. '* Complemental males " — pigmies attached to hermaphrodites —
occur in two groups.
9. The conditions of hermaphroditism. Commonest in sedentary,
sluggish, parasitic forms.
10. Hermaphroditism is primitive ; the unisexual state is a subsequent
differentiation. Or, Unisexuality is primitive; the hermaphrodite state
secondary. Possibly both suggestions may be true.
LITERATURE.
See already cited works of
Gegenbaur, Hensen, Hertwig, Hatchett Jackson and Rolleston,
passim.
Beard, J. — The Sexual Conditions of Myzostoma glabrum. MT. Zool.
Slat. Neapel., XIII., 1898, pp. 293-324, i pi.
Bourne. — On Certain Abnormalities in the Common Frog. i. The
Occurrence of an Ovotestis. Quart. J. Micr. Sc, XXI \^
Brock. — Morph. Jahrb., IV. Beiirage zur Anatomic und Histologic der
Geschlechtsorgane dcr Knochenfische.
Giles.— Quart. Tourn. Micr. Sci. 1888.
Haacke, W. — ^Die Bedeutung und die Folgen der Inzestzucht. Biol.
Centrlbl., XV., 1895, pp. 44-78.
Kknnel, J.— Schrift. Nat. Ges. Jurjeff (Dorpat), IX., 1896, pp. 1-64.
LaulanU, F.— Comptes Rendus, CI. (1885), pp. 393-5.
Marshall, A. Milnes. — On Certain Abnormal Conditions of the
Reproductive Organs in the Frog. Journ. Anat. Physiol., XVIII.
pp. 121-44.
MttEHAN, T. — On Self- Fertilisation and Cross-Fertilisation in Flowers
Penn. Monthly, VII. (1876), pp. 834-43.
Montgomery, T. H. — On Successive Prolandric and Proterogynic Her
maphroditism in Animals. Amer. Naturalist, XXIX., 1895, pp
528.36.
Pklseneer, p. — L'hermaphroditisme chez les MoUusques. Arch. Biol.
1895, pp. 33-62, 2 pis. Quart. Journ. Micr. Sci., CXLV., 1894
pp. 19-46, 2 pis.
86 THE EVOLUTION OF SEX.
PflOger, E.— Archiv. ges. Physiol., XXIX.
Prouho, II. — Dioicite et Hermaphroditisme chez les Myzostomes. ZooL
Anzeiger, XVIII., 1895, pp. 392-5.
Simpson, J. Y. — Todd's Cyclopaedia of Anatomy and Physiology. Art.
Hermaphroditism, pp. 684-738 (1836-9).
Spengel. — Arb. Wiirzburg, III., 1876. Ueber d. Urogenital System dcr
Amphibien.
Zwitterbildung bei Amphibien. Biol. Centrlbl., IV., 8, cf. 9.
Sutton, J. B. — Hypertrophy and its Value in Evolution. Proc. Zool.
Soc., London, 1888, pp. 432.
General Pathology. London, 1886.
Wheeler, W. M. — The Sexual Phases of Myzostoma. MT. Zool. Stat.
Neapel., XIL, 1896, pp. 227-302. ZooL Anzeig., XXII. (189;),
pp. 281-8.
CHAPTER Vn.
The Ultimate Sex-Elements {General and HtUorieat).
In our analysis of sex- characters we have followed the general
course of biological history. We first passed from the form
and habit of a male or female organism to the structure and
functions of the sexual organs. In discussing hermaphroditism,
we had occasion to refer to a third step of biological analysis,
that which involves an investigation of the properties of the
cxiEma) pcroui isix or lona pelluoda (lO, uul folUcuLu'ccllil.fA
tissues. Now it is necessary to penetrate deeper, namely, to
the sexcells. After these have been considered we shall be
in a better position to re-ascend to some of the problems of
reproduction.
8& THE EVOLUTION OF SE3t.
§ 1. The Ovum Tluory. —li is now a commonplace of
observation and established fact, that all organisms, reproduced
in the ordinary way, start in life as single cells. We see insects
laying their ova upon plants, or fishes shedding them in the
water, and may watch how these cells, provided they be
fertilised, give rise eventually to the adult organisms. Con-
veniently in the ordinary frog-spawn from the ditch, we can
read, what was for so long a riddle, how development proceeds
by successive cell-divisions and by arrangement of the multiple
results. Readily seen in many instances, it is true of all cases
of ordinary sexual reproduction, that the organism starts from
the union of two sex-cells. In other words, it is with the division
of a fertilised ovum that development begins.
This profound fact, technically known as the "ovum
theory," has been not unjustly called by Agassiz " the greatest
discovery in the natural sciences of modern times." ^^'e shall
the better realise the magnitude of the difference which its
recognition has introduced into biology, if we briefly review
the history.
§ 2. The History of Embryology: Evolution and Epigenesis,
— The development of the chick, so much studied in em-
bryological laboratories to-day, was the subject of inquiry two
thousand years ago in Greece. Some of the conspicuous marvels
of reproduction and development were the subjects of persistent
but fruitless speculation throughout many centuries. It was
only during the scientific renaissance of the seventeenth century
that the inquiry became more keen and precise, and began to
rely to some extent at least on genuine observation.
{a) Harvey (1651), with .the aid of magnifying glasses
{perspecillce\ demonstrated in the fowl's egg the connection
between the cicatricula of the yolk and the rudiments of
the chick, and also observed some of the stages of uterine
life in mammals. More important, however, were his far-
sighted general conclusions, — (i.) That every animal was pro-
duced from an ovum (ovum esseprimordium commune omnibus
animalibus)\ and (2.) Tlxit the organs arose by new formation
(epigenesis\ not from the mere expansion of some invisible pre-
formation. In this generalisation, without however any abandon-
ment of the hypothesis of spontaneous generation of germs, he
strove, as he said, to follow his master Aristotle, and was in so
doing as far ahead of his contemporaries as a strong genius
usually is. Before Harvey, the observational method had
THE ULTIMATE SEX-ELEMENTS. 89
Indeed begun. Thus, as Allen Thomson notes, Volcher Coiter
of Groningen (1573), along with Aldrovandus of Bologna, had
watched the incubated egg in its marvellous progress from day
to day. Fabricius ab Aquapendente (1621) had also studied
the changes in the incubated egg, and the stages of the
mammalian foetus. In keenness of vision, Harvey was far
ahead of these.
(if) Malpighi (1672), using a microscope with phenomenal
skill, traced the embryo back into the recesses of the cicatricula
or rudiment, but again missed a magnificent discovery, and
supposed the rudiments to have pre-existed in the egg. In
1677, Leeuwenhock was led by Hamm to the discovery of
the spermatozoa; and this was followed up, though not to
much profit, by Vallisneri and others. Steno, too, in 1664,
had given the ovary its present designation ; and De Graaf had
interpreted the vesicles of this organ, which now bear his name,
as for the most part equivalent to the ova which he had dis-
covered in the oviduct. Needham (1667), Swammerdam (1685),
and J. van Heme, also contributed items of information not
then appreciated in their real relations.
(c) The Theory of Preformation — Ovists and Animalculists,
— In the early part of the eighteenth century, the embryological
observations of investigators, e.g, Boerhaave, were summed up
in the conception that development was merely an expansion
or unfolding of a pre-existent or preformed rudiment within the
egg. Harvey had indeed striven for an opposite conclusion,
but his view was negatived, as we have seen, by Malpighi's
failure to trace the embryo beyond the rudiments of the
cicatricula.
The notion of a preformed rudiment, thus suggested by
Boerhaave, Malpighi, and others, rapidly became the prevalent
theory. In so far as it emphasises one side of the facts, it is
bound in modified form so to remain. Leibnitz, Malebranchc,
and others found it to fit in better with their cosmic concep-
tions than the older view of Aristotle had done, and welcomed
it accordingly.
The positions occupied by the physiologist Haller well
illustrate the alterations of opinion. As Allen Thomson points
out in his article "Embryology," in the Encyclopedia
Britannica^ "Haller was originally educated as a believer in
the doctrine of * preformation ' by his teacher Boerhaave, but
was soon led to abandon that view in favour of ' ei)igenesis * or
90 THE EVOLUTION OF SEX,
new formation. But some years later, and after having been
engaged in observing the phenomena of development in the
incubated egg, he again changed his views, and during the
remainder of his life was a keen opponent of the system of
epigenesis, and a defender and exponent of the theory of
* evolution,' as it was then named."
The preformation theory found more and more definite
expression in the works of Bonnet, Buffon, and others. It is
now necessary to sum up its main propositions.
The germ, whether egg-cell or seed, was believed to be a
miniature model of the adult. "Preformed" in all trans
parency the organism lay within the egg, only requiring to
be unfolded. Many affirmed, that before fertilisation there
lay within the fowl's ovum an excessively minute but complete
chick. They compared the germ to a complex bud, which
hides within its hull the floral organs of the future. Harvey
had said, "the first concrement of the future body grows,,
gradually divides, and is distinguished into parts ; not all at
once, but some produced after the others, each emerging in its
order." Very different was Haller's first and last utterance,
"There is no becoming; no part of the body is made from
another, all are created at once." This was obviously a short
and easy method with embryology, if the organism was literally
preformed in the germ, and its development simply a growth
and an unfolding.
But this was not all. The germ was more than a marvellous
bud-like miniature of the adult, it necessarily included in its
turn the next generation, and this the next — in short all future
generations. Germ within germ, in ever smaller miniature, after
the fashion of an infinite juggler's - box, was the corollary
logically appended to this theory of preformation and unfold-
ing, — of evolution^ as it was then called, in a very different but
more literal sense from that in which we now use the word.
A side controversy of the time arose between two schools,
who called each other "ovists" and "animalculists." The
former maintained that the female germ element was the more
important, and only required to be as it were awakened by the
male element to begin the process of unfolding. The animal-
culists, on the other hand, asserted the claims of the sperm to be
the bearer of the miniature nest of organism within organism,
and supposed that it only required to be fed by the ovum to
enlarge and unfold the first of the models which it concealed.
THE ULTIMATE SEX-ELEMENl^S.
91
{d) Wolffs s Reassertion of Epige nests. — The above ingenious
construction was rudely shaken down, however, in 1759, when
Caspar Friedrich Wolff showed, in his doctorial dissertation, the
illegitimacy of the suppositions which lay at the root of the
The first stages of development in a numl^cr of animalK. .(,
Sponge, Coral, Earthworm, or Starfish ; B, Crayfish or
other ArthroDod ; C, Tunicate, Lancelet, &c. ; /?, Frog
or other Ampnibian.
X. Fertilised ovum ; 3. Seemenied ovum, a ball of cells, morula,
or blastosphere ; 3. 'I'he same, after further division or in
section ; 4. The g.istrula stage.
preformation theory. He traced the chick back to a layer of
organised particles (the familiar cells of to-day), in which there
was no likeness of the future embryo, far less adult. Mors
94 THE EVOLUTION OF SEX.
cular change or metabolism. On the one hand, more or less
simple dead matter or food passes into \\(e by a series of
assimilative ascending changes, with each of which it becomes
molecularly more complex and unstable. On the other hand,
the resulting protoplasm is continually breaking down into more
and more simple compounds, and finally into waste products.
The ascending, synthetic, constructive series of changes are
termed "anabolic"; and the descending, disruptive series, "Icata-
bolic." Both processes may be manifold, and the predominance
of a particular series of anabolic or katabolic changes implies
the specialisation of the cell.
The figure (a, on p. 93) represents ihe complex unstable protoplasm >s if
occupying ihe summit of a double flight of steps; it is formed up the
analxilic steps, i( breaks up and descends by the katabolic. The lower
ligare (b) is a projection of the other, its conve^ent and divergent lines
sCTving to represent the various special lines of anabolism and katabolism
respectively, and the definite component substances ("anastates" and
"katastates") which it is the task of the chemical physiologist to isolate
and interpret.
From a general physiological point of view it matters little if we make
s slightly different aasumption, naniely, that protoplasm does not exactly
share in the twofold process of metabolism, but is a substance per se, acting,
like a ferment, on the complex materials around it. Even if we suppose,
as some do, that there is no such substance as protoplasm, life being the
THE ULTIMATE SEX-ELEMENTS.
§ 5. Protozoa and Metazoa. — It has been emphasised above
that every multicellular organism, reproduced in the ordinary
way, starts from a fertilised ovum, from what may be fairly
called a single cell. Sponge, butterfly, bird, and whale begin,
in a sense, at the level of the simplest animals or Protozoa,
which (with the exception of some which form colonies) remain
always unicellular. The simplest organisms leave off where
the higher plants and animals begin, j.e., as single corpuscles
of living matter. They correspond, in fact, to the reproduc-
tive ceils of higher animals, and may be called, according to
their predominant character, protova and protosperms. A
fertilised ovum, as we have seen, proceeds by division to form
a "body"; the Protozoon remains, with few exceptions, a
single cell, in which there is obviously no distinction between
reproductive elements and the entire organism.
Reference will have to be made to the Protozoa in three
connections, which may be here simply noted: —
(a) In their chief groups, and in the stages of their life-
histories, they express phases in the same cell cycle which
recurs in higher forms in the component elements of the body,
and in the reproductive cells. The contrast, in other words,
between an infusorian and an amceba, between the ciliated
96 THE EVOLUTION OF SEX.
and amGeboid stage in the life-history of many forms, is a fore-
cast of the contrast between a ciliated cell and a white blood
corpuscle, between a mobile spermatozoon and a young ovum.
A predominance of the same protoplasmic processes is the
basal interpretation of the similarities of form.
{b) It is among the Protozoa that we must look, if we hope
to understand the origin and import either of "male and
female/* or of fertilisatioa
{c) Among the loose colonies which some Protozoa form,
and which bridge the gulf between the unicellular animals and
the Metazoa, there is seen the beginning not only of the for-
mation of a "body" (see figs, on pp. 94, 95), but also the
setting apart of special reproductive cells. On this point
more emphasis must be laid. The ordinary Protozoon is a
single cell, and forms no body. It divides indeed, and multi-
plies accordingly, but the products of division go asunder,
whereas in the segmentation of the ovum they remain con-
nected. In most Protozoa, there is continual self-recupera-
tion; in most, division occurs without any loss; in most, there
is no distinction between parent and offspring; in most, as
there is no body, there is no death. Thus it is that, with one
weighty caution to be afterwards noted, it seems justifiable to
speak with Weismann and others of the " immortality of the
Protozoa." In a certain sense too, as we shall see, it is justifi-
able to speak of the immortality of the reproductive cells in
higher animals. The body dies, but the reproductive cells
escape, before its death, to live on, as new organisms, enclosing
new sets of reproductive cells. Again there is similarity
between the Protozoa and the reproductive cells.
But in some of the loose colonies {e.g., Volvox\ we see the
beginning of the change which introduced death as a constant
phenomenon (see fig. p. 139). The cell, which starts one of
these colonies, divides; the products of division, instead of
going apart as usual, remain connected; a loose body of many
cells is thus formed. In this cluster of cells, certain elements
are in turn set apart and eventually adrift, as reproductive
cells. They start new colonies, and thus we are introduced to
what is constant in higher animals. The only marked differ-
ences are — {a) that the body of the Metazoon is more than a
loose colony of cells; {b) that the reproductive elements are
usually liberated from some definite region or organ ; and (r) that
they are more markedly differentiated as male and female cells.
THE ULTIMATE SEX-ELEMENTS. 97
§ 6. General Origin of the Sex-Cells, — Except in the lowest
invertebrates, the sponges and coelenterates, the reproductive
elements almost always arise in connection with the middle
layer (mesoderm or mesoblast) of the body.
Neither in sponges nor in aclenterales is there a middle layer exactly
comparable to the mesoderm of higher animals; the less definite middle
stratum is now frequently termed a mcsoglaia. In sponges, we already
mentioned that the reproductive cells simply arise here and there among
the other elements of the stratum. The ova are highly nourished meso-
glseal cells; the primitive male cells, which divide into numerous minute
spermatozoa, are the reverse.
In ccelenterates the phenomena are of much interest; the origin of the
sex-cells is very diverse. Some time ago considerable emphasis was laid,
by E. van Beneden and others, on the fact that, in certain Hydrozoa, *' the
ova are derived from the endoderm, and the sperms from the ectoderm."
Thus Gegenbaur, accepting this, remarks that in such cases **the
endo<1crm is the female, and the ectoderm the male germinal layer."
Such a generalisation, if established, would be plausible enough, seeing
that the inner or endoderm layer is the more nutritive or anabolic of the
two. A controversy, however, soon arose, the result of which was to over-
throw the generalisation. In hydra, we have already noticed that both
products arise from the ectoderm ; the same was shown by Ciamician to be
true of Tubularia ntesembryofUhemum ; while in the Ettdendriuni ramosum
the ova appeared to arise from the ectoderm^ and the male elements from
the endoderm^ the very reverse of Van Beneden's conclusion. The matter
was settled, so far as the general facts are concerned, by Weismann, who
established the fact of active migration of the elements from one layer to
another. He has since been followed by other investigators, {a) The sex-
elements, l)oth male and female, may appear first in the endoderm, whether
they originate there or not, and from this inner layer they migrate to the
ectoderm, where they ripen, {b) In rare cases they even ripen in the
endoderm. {c) Very commonly the sex-cells originate in the ectoderm and
ripen there, or they may pass thence into the endoderm and back again to
the ectoderm. (</) In the medusa of Obelia^ the ova appear to ripen partly
in both layers. These facts, a convenient summary of which will be
found in Hatchett Jackson's erudite edition of Rolleston's *' Forms of
Animal Life,*' show plainly enough how varied are the origin and history
of the sex-cells in these forms.
The colonial hydroids typically produce well-marked reproductive
individuals or sexual zooidf, set free as ^* swimming-bells" or medusoids
(in a process to be afterwards descrilied under ** Alternation of Genera-
tions "). In these the reproductive elements are typically developed. But
in varying degrees these medusoids have degenerated, and are frequently
not only not liberated, but lose their characteristic features, and Ix^come
mere reproductive buds. In these buds the sex-cells are normally
developed. But it very frequently happens that they arise more or less in
the body of the asexual vegetative hydroid. They ripen early, and sub-
sequently migrate to their proper place; the asexual stage incorporating
more and more of the originally separate sexual generation. Weismann has
emphasised the value of this early ripening as an advantage to the race,
7
98 THE EVOLUTION OF SEX.
lessening the danger of its extinction ; and this has doubtless to be con-
sidered. But important as all such considerations are, they cannot dispense
with an enquiry into the physiology of the facts.
§ 7. Early Separation of Sex-Ceils, — Having noted the
general fact of mesodermic origin, and some of the interesting
phenomena observed in coelenterates, we shall not further
pursue the subject except as regards one question, the period
at which the reproductive cells make their appearance. This
is sometimes early, sometimes late; and it is not yet decisively
known in how many cases early separation occurs, nor how far
the fact is of much significance.
In the case of a well-known fly, Ciiironomus^ Professor Bal-
biani, unprejudiced by any theory of heredity, observed the
following facts: — Before the segmentation of the egg had at all
advanced, before what embryologists call the blastoderm was
more than incipient, two cells were observed to be set apart
externally. (These had nothing whatever to do with the polar
globules seen in most ova at maturation.) The development
proceeded apace, but the isolated cells took no share; they
may be presumed to have retained intact the characters which
they received when first divided off from the ovum. At a
certain stage, however, the isolated cells sank inwards, took up
an internal position, became the rudiments of the reproductive
organs. Here then, at an early stage, before differentiation is
marked, the reproductive cells are set apart. They must there-
fore preserve much of the character of the parent ovum, and
hand on the tradition intact by continuous cell-division to the
next generation.
In other words, in the preceding case, at a very early stage
in the embryo, the future reproductive cells are distinguishable
and separable from the body-forming cells. The latter develop
in manifold variety, into skin and nerve, muscle and blood, gut
and gland; they differentiate, and lose almost all protoplasmic
likeness to the mother ovum. But the reproductive cells are
set apart; they take no share in the differentiation, but remain
virtually unchanged, and continue unaltered the protoplasmic
tradition of the original ovum. After a while they, or their
division-products rather, will be liberated as reproductive cells.
These in a sense will be continuous with the parental germ.
Their protoplasm will be more or less identical. The original
ovum has certain characteristics, a h c x y z; it divides, and all
its cells must at first more or less share these characteristics;
THE ULTIMATE SEX-ELEMENTS. 99
the body-cells lose some of them, the insulated reproductive
cells retain them all. The ovum of the next generation has
thus also the characteristics a b c x y z, and must therefore
produce an organism essentially like the parent.
An early isolation of the reproductive cells, though rarely so
striking as in Chironomus^ has been observed in many cases, —
e,g,y in some other insects, in the aberrant worm-type Sagitta^
in leeches, in threadworms, in some Polyzoa, in some small
crustaceans known as Cladocera, in the water-flea Moina^ in
the parasitic Hymenopteron Platygaster^ in some arachnoids
(Phalangidse), in the bony fish MicromeUus aggregatus. As
the series is ascended, the reproductive organs seem to be later
in making their appearance, at least they are only detected at
a later stage. It must also be pointed out that, in cases of alter-
nation of generations, an entire asexual generation, or more than
one, may intervene between one ovum and another.
Perhaps the most striking of all the cases of the early segre-
gation of the lineage of germ-cells is that described by Boveri
in Ascaris megalocephala^ the threadworm of the horse. He
was able to trace back the germ-cells continuously to the two-
cell stage. At the very first cleavage the distinction between
somatic and reproductive is established. One of the first two
cells is the ancestor of all the cells of the body; the other is
the ancestor of all the germ-cells. " Moreover, from the out-
set the progenitor of the germ-cells differs from the somatic ceils
not only in the greater size and richness of the chromatin of its
nucleus but also in its mode of mitosis, for in all those blasto-
meres destined to produce somatic cells a portion of the
chromatin is cast out into the cytoplasm, where it degenerates,
and only in the germ-cells is the sum-total of the chromatin
retained^' — (E. B. Wilson, "The Cell in Development and
Inheritance," 1896, p. iii.)
§ 8. Body Cells and Reproductive Cells. — Various naturalists
have insisted on the contrast hinted at above, between the cells
of the embryo which go to form the body and those which are
set apart as reproductive organs.
{a) As early as 1849, Owen noted that, in the developing
germ, it was possible to distinguish between cells which became
much changed to form the body, and cells which remained
little changed and formed the reproductive organs. This view,
as Brooks points out, he unfortunately afterwards departed from
in his "Anatomy of the Vertebrates."
100 THE EVOLUTION OF SEX.
{b) In 1866, Haeckel connected reproduction with discon-
tinuous growth, and insisted upon the material continuity
between parent and offspring. Somewhat later, both he and
Rauber drew a clear contrast between the somatic and repro-
ductive elements, between the "personal" and "germinal"
portions of the embryo, or between the body and the sex cells.
{c) W. K- Brooks, in 1876 and 1877^ again drew attention
to this significant contrast.
{d) Yet more explicit, in 1877, was the ingenious Dr
Jager, now better known in a very different connection, and
a few of his sentences well deserve to be quoted. Referring
to a previous paper, he writes as follows : — " Through a great
series of generations, the germinal protoplasm retains its specific
properties, dividing in every reproduction into an ontogenetic
portion, out of which the individual is built up, and a phylo-
genetic portion, which is reserved to form the reproductive
material of the mature offspring. This reservation of the
phylogenetic material I described as the continuity of the germ-
protoplasm, Encapsuled in the ontogenetic material, the phylo-
genetic protoplasm is sheltered from external influences and
retains its specific and embryonic characters."
(e) In an exceedingly clear manner, to which sufHcient
attention seems hardly to have been accorded, Galton, in 1876
and at other dates, as again more indirectly in his work on
" Natural Inheritance," drew attention to the corxtrast between
the gem mules of the ovum (stirp) which go to form the body,
and those which, remaining undeveloped, form the sex-cells.
" The developed part of the stirp is almost sterile *' (z.^., with-
out influence in heredity); " it is from the undeveloped residue
that the sexual elements are derived."
(/) Lastly, in 1880, Nussbaum, in an elaborate investiga-
tion on the differentiation of the reproductive cells, drew
emphatic attention to some cases of their early separation, and
reasserted Jager's conception of a continuity of germ-protoplasm.
In this survey, however, we do not pretend to decide the
difficult question of priority in the enunciation of this con-
ception. Like many other generalisations, it appears to have
arisen all but simultaneously in many minds.
(g) It is to Weismann, however, that biology is particu-
larly indebted for his vindication and elaboration of the
doctrine of genetic continuity.
§ 9. JVeismann's Theory of the Continuity oj the Germ-
Protoplaiin. — ^In some cases referred to in a foregoing paragrapli,
it is possible to trace a direct cellular con-
tinuity, first of all, between the ovum and
early separated reproductive rudiments,
secondly, between the latter and the future ",
ova and sperms. There is not only cellular
continuity between the ovum which gives rise
to parent, and the ovum which gives rise to
offspring, — that the cell-theory demands, —
but there is a continuity in which the char- I '
acter of the original ovum is never lost by
difTerentiation. In fact, there is a continuous
chain of reproductive cells quite apart from »_
the body cells. It is in this sense that some I
of the authors quoted have spoken of the
continuity of the germ-^«//(. This is certainly ,
true for some cases. If it were true for all, j .
the problems of reproduction and heredity
would be much simpler than they at present I
appear to be. f
For in the present stale of our knowledge
we can only speak of the continuity of the \
reproductive (ells, in exceptional or rather in I
a small minority of cases. Alike in the higher |
vertebrates and the lowly hydroids, the re- "
productive cells may appear late. After the I
differentiation of the vertebrate embryo has i
progressed far, or the life of the polyps con- <
tinued for long, the germ-cells make their <
apptearance; and though we know of course ,
that they are descendants of the original tik rei;.iion between re-
ovum, yet we must allow, with Weisnianii, [^^b^'J!' jj|; ^.
that in the form of special cells they are now .inuou* chain of IhmI
for the first time to Ije detected. Therefore, T"!;.?^'^.'!.-
Weismann says, "a continuity of gcxm-ce//s ""• J,''"Jj^ "i'f.i];;
is now for the most part no longer demon- w™" 'lucVei/iv"
slrahle." du^™« ^h fZ
Yet there is nothing that Weismann «iiion, » sptrmai*
more strongly insists upon, than the reality Kt^raiJd'ovuin'ii also
of continuity between ovum and ovum, indioied.
In what does it consist, if a chain of ovum-like celts
is only true of a minority of organisms? It consists,
102 THE EVOLUTION OF SEX.
according to Weisniann, in the " Keimplasma " or germ-
plasm.
The germ-plasm is the distinctive part of the nucleus of
the germ-cell. It has an extremely complex, and at the same
time persistent, structure. It is the substance which enables the
germ-cell to build up an organism; it is the architectural living
matter, and the immortal bearer of all properties transmitted in
inheritance. " In every development," according to Weismann,
•*a portion of this specific germ-plasm, which the parental
ovum contains, is unused in the upbuilding of the offspring's
body, and is reserved unchanged to form the germ-cells of the
next generation. . . . The germ-cells no longer appear as
products of the body, at least not in their most essential part —
the specific germ-plasm; they appear rather as something
opposed to the sum-total of body-cells; and the germ-cells of
successive generations are related to one another like genera-
tions of Protozoa." But the continuity is rarely kept up by a
chain of undifferentiated reproductive cells; it depends upon
the continuance and unchanged persistence of part of the
original germ-plasm.
It need hardly be pointed out that the conception of the
germ -plasm is theoretical. For just as we cannot point to any one
portion of the cell and say this is protoplasm — the living matter
— and nought else; so we cannot demonstrate the germplasm^
even if we accept the current view that it has its basis in the
chromatin of the nucleus. The theory is a conceptual inter-
pretation, and must be tested by its power of fitting facts.
THE ULTIMATE SEJC-ELtMENTS. loj
SUMMARY.
The progressive analysis through organism, organs, tissues, and cell^',
to the living matter itself.
1. The Ovum-theory. — Every multicellular organism, reproduced in the
ordinary way, arises from a fertilised ^g-cell, and development proceeds
by cell-division.
2. Epigenesis and Evolution. — History of the different views taken of
the development of the organism; ancient speculations. The scientific
renaissance, (a) Harvey's prevision of the ovum-theory, and emphasis
upon "epigenesis." {d) Observations of Malpighi and others, mostly
against Harvey's view, (c) The theory of preformation,— of a nest of
miniature models within the egg, only requiring to be unfolded in succes-
sive generations; Ovists versus Animalculists. [d) Wolff's reassertion of
*' epigenesis," the foundation of modern embryology; his exaggeration of
the simplicity of the germ, (e) Wolff's successors.
3. The Cell-Theory. — All organisms have a cellular structure and a
cellular origin.
4. A protoplasmic basis now being laid. The *' germ-plasm " more
important than the egg-cell. All to be interpreted in terms of protoplasmic
changes.
5. The contrast between Protozoa and Metazoa. — The making of a
** body " as dbtinct from reproductive cells.
6. General origin of the sex-cells, indefinite in sponges, variable in
coelenterates, generally from the mesoderm in higher animals.
7. Early separation of the reproductive cells to be seen in a minority of
cases.
8. The contrast between somatic and reproductive cells, and the con-
tinuity of the latter; Owen, Haeckel, Rauber, Brooks, Jager, Galton,
Nussbaum, Weismann.
9. Weismann's theory of the continuity of the gcim-p/asm (a specific
nuclear matter), as opposed to continuity by a lineage of undifferentiated
germ-ce/Zs, which is known to occur only in a minority of organisms.
LITERATURE.
For relevant literature and further details, consult the Text-books of
Balfour, Iladdon, and Ilertwig; also,
Delage. — Heredity, etc. 1895.
Gbddes, p. — Encyclopedia Britannica articles already referred to; also
Morphology, t'M.
IIensen, v.— Op. cit.
M'Kendrick, J. Cf. — Text -book of Physiology. London, 1888.
Thomson, J. A.— Arts. Cell and Embryology, new edition of
Chambers's Encyclopedia.
— — — History and Theory of Heredity. Proc. Roy. Soc. Edin., 18S8.
Waldeyer, W. — Die Karyokinese, &c. Arch. Mikr. Anat., 1888.
Weismann.— (9//. cit,
Wilson, E. B. — The Cell in Development and Inheritance. New edition,
190a
Zoological Record, General Subjects: Cell, Oogenesis, &c., since 18S6.
CHAPTER VIII.
The Egg-Cell or Ovum.
In the preceding chapter we sketched ihe history of the
" ovum -theory," which expresses the now familiar fact that
every organism, reproduced in the ordinary way, develops from
a fertilised eggcell. The exceptions are the unicellular plants
and animals, all forms of asexual reproduction, and the special
(•i) ill ■ llHia coil, and the pio(apl:ianiic nelniffk If)
I'lund jiboul.— Fram Qirnny.
case of parthenogenesis (where the egg-cell is not fertilised).
It is now necessary to attend more carefully to the essential
characters and history of this "primordium commune," the
starting-point of the individual life.
g J. S'rKCtu-t of the Ovum. — The ovum presents all the
OVUM. 1 05
essential Teatures of any other animal cell. There is the cell-
substance, consisting in part of genuine living matter or proto-
plasm; and there is the nucleus, or "germinal vesicle," which
plays such an important part in the ripening, fertilising, and
subsequent division of the cell.
Besides the living matter, there are simpler substances,
especially in many cases a reserve capital oF yolk nutriment
for the future embryo. The modern masters of microscopic
technique have detected many marvels in the egg-cell, into
Ovum ofa Tlircadworm (.-Iscar/l), ihowing (a)lhe chromalin
eleiriFnt^ of Ilic nuEkui, and the appdmiicc oC the
jurrounding yolk.— From Oimoy.
which we cannot nt present enter, but it is important to
recognise clearly that although the ovum is in a sen.se simple,
being a single cell, it is not structureless like white of egg.
About details there is great diversity of opinion, but all are
agreed that the ovum has "organisation."
When Purkinje, in 1825. discovered the nucleus of the
fowl's egg, he probably had little idea that the minute
"vesicle" to which he directed the attention of investigators was
in reality an intricate microcosm. Little more than ten years
lo6 THE EVOLUTION OF SEX.
elapsed before R. Wagner began to complicate matters by the
discovery of the nucleolus or germinal "spot" within the
"vesicle." We now know that the nucleus has not only a
very complex structure, but in a sense a strange internal life of
its own.
The nucleus, when quiescent, often lies in a little nest or
chamber within the cell -substance, and is limited from the
latter by a more or less distinct nuclear membrane, which dis-
appears as the period of activity begins. Inside this mem-
brane, it is often possible to distinguish one or more of the
aforesaid nucleoli, lying in a more fluid material often called
the "nuclear sap." About these nucleoli and bodies more or
less like them, about the reasons for their variable number and
form, very little that is certain can be said. Much more
important is the most conspicuous constituent of the nucleus,
a system of strands, coils, or loops, which stain deeply with
various dyes, and are therefore known as the chromatin ele-
ments. In contrast thereto, the less stainable but perhaps
equally essential constituents of the nucleus are distinguished
as achromatin.
The chromatin elements in the resting nucleus are oftenest
arranged in a manifold coil, like a loosened ball of twine, while
in other cases they appear rather as a living network. Whether
the coil be continuous, as Van Beneden and others describe,
or interrupted, as Boveri and others maintain, is subsidiary to
the more striking fact, that in the state of activity the number
and disposition of the dislocated or loosened parts of the coil
remain definite and orderly, and that their behaviour is so like
that of minute independent individualities that any rough-and-
ready account of the mechanics of cell division must at once
be ruled out of court. It is within the chromatin substance
too that the germ-plasm, on which Weismann and others have
so much insisted, is believed to have its seat. There can be
no doubt that the nucleus is a very important factor in the life
of the cell. A non-nucleated fragment cut from a Protozoon
may show irritability and contractibility, but it cannot feed,
and soon dies. Boveri and Delage have shown in sea-urchins,
&c., that an ovum-fragment without a nucleus may be fertilised
and may develop into a larva, so that it seems as if the sperm-
nucleus may in certain cases suffice, but no instance is known
of development without any nucleus.
§ 2. Grmvth of the Ovum. — When the ovum is very young,
THE EGG-CELL OR OVUM. I07
it very frequently presents the features of an amceboid cell.
In some cases this phase persists for a longer time, as in the
ovum of hydra, which in all essentials is comparable to an
amoeba. Even in the simplest animals, however, the amoeboid
phase constantly shows a tendency to pass into greater quies-
cence, to become in fact more or less encysted. So is it with
ova, which though at first often resembling various forms
of amoeboid cells, tend more or less quickly to pass into
the encysted phase. The protoplasm no longer flows out in
irregular ever-changing processes, but is gathered up into a
sphere, rounded off, and surrounded by a more or less definite
envelope. This transition from a state of relative equilibrium
between activity and passivity, to one in which passivity un-
doubtedly preponderates, is associated with an increase of
nutriment and reserve-products. The ovum feeds, becomes
heavy with stored capital, becomes less active, and more en-
cysted in consequence.
§ 3. Yolk, — The essential part of an egg-cell is always small,
though even in this there are great differences. The nucleus,
for instance, in the large eggs of amphibians, reptiles, and
birds, may be detected with the unaided eye; while in other
cases, such as sponges, the entire ovum is very minute. Yet
every one knows that eggs vary enormously in size. The egg
of a skate is very much larger than the egg of a salmon ; and
the egg-shell of the extinct giant bird of Madagascar (^-^Py-
ornis) is big enough to hold the contents of one hundred and
fifty hens* eggs. Similarly the contrast between the eggs of
ostrich and humming-bird is, as one would expect, extremely
striking. Yet the eggs of whales are "not larger than fern-
seed," and the same is true for most mammals, except the
very lowest. The differences in size, when very striking, are
due not so much to any marked disproportion in the essential
parts of the ova, but to certain extrinsic additions. The most
important of these is the yolk, which serves as nutritive
capital for the embryo or young animal. Besides the yolk, we
have also to take into account the frequent pigment, so familiar
in frog spawn, the albumen well seen in the white of birds'
eggs, various forms of protective and attaching viscid material,
and, lastly, more or less elaborate egg envelopes or shells.
The most important, however, is the yolk, and in regard to its
origin and disposition a little must be said.
The e^ has its nutritive capital increased in three different
10
8 THE EVOLUTION OF SEX.
ways: — (a) Very generally it feeds on the nutritive substances
in the general lymph or vascular fluid of body, (fi) At the
same time, or in another case, it avails itself of the dSdris of
surrounding cells. In many instances, e,^., in the minute
ovary of hydra, in the ovary of Tubulariay or in the ovarian
tubes of insects, the ovum is but the surviving competitor
among a crowd of surrounding cells, which to start with were
all potential ova. This is an often forgotten chapter in the
struggle for existence, — the struggle between germ-cells.
There is a struggle between potential ova; there is also
enormous elimination among the spermatozoa, even after they
come to close quarters with the ovum. Many are almost suc-
cessful, but in most cases only one fertilises, f.^., survives.
And even after the eggs begin to develop there is often
elimination apart from enemies, thus it is stated that only
about a third of the eggs of the New Zealand " lizard "
(Sphenodon or HaUeria) ever hatch, (c) In the third place,
and this is the rarest form, the egg-cell acquires a store of
food-material from a special yolk gland, as in many of the
lower " worms."
The yolk, gained in one of the ways mentioned above, is
more or less readily distinguished from what is often called
the formative protoplasm. Out of the latter the embryo is
built up, while the yolk has for the most part only a secondary
and nutritive rdU, We cannot enter here into a discussion of
the difficult embryological question as to the extent in which
the yolk ever shares in directly contributing to embryonic
structures. The possibility of distinguishing between for-
mative protoplasm and the nutritive mateiial, depends on the
quantity of the latter that is present, and on the way in which
it is disposed, (a) When there is not much of it, as in the
small ova of mammals and many invertebrates, the yolk
material is diffusely distributed. Then the ovum undergoes
complete segmentation, (b) In the frog's ovum, on the other
hand, there is a large proportion of yolk, which has especially
accumulated in the lower hemisphere of the cell, while the
darker half includes the truly formative protoplasm. In this
case too the egg divides as a whole, but the divisions go on
much more rapidly in the upper hemisphere, and it is there
that the embryo is really formed. (<r) A distinct mode of
yolk arrangement occurs in arthropods (crustaceans, insects,
&c.), where the centre, not a pole, of the ovum is occupied by
THE EGG-CELL OR OVUM.
109
the nutritive material In this case the formative protoplasm
divides round about the nutrient core, (d) In the majority of
fishes, in reptiles, and in birds, the eggs show a much more
marked polar accumulation of yolk. On the top of a large
mass of nutritive material, the specifically lighter formative
protoplasm lies like a tiny drop, and in those cases the division
A
^J^
D"
The relation between the disposition of the yolk and the mode
and discoidal segmentation.
of the ovum is very partial, — that is, it is mainly restricted to
the upper formative region. It is thus to be noted that the
quantity of yolk present, and its diffuse, polar, or central
arrangement, are associated with striking differences in the
degree and symmetry of the segmentation.
no THE EVOLUTION OF SEX.
§ 4. Composite Ova, — We have emphasised the fact that the ovum must
be regarded as a single cell. To this a definite but pedantic objection has
been raised. In some parasitic flat worms there occur what have been
called compound ova. A minute single cell arises, as usual, in the ovary,
but in the course of its somewhat intricate history this becomes associated
with several nutrient cells derived from the yolk-gland. These surround
the original ovum, so that the whole now consists of several cells, fiut it
must be noticed that only the central cell — the ovum proper— is fertilised,
and that it contains all the formative protoplasm. Those that surround it
are wholly nutritive ; they eventually break up, and are absorbed.
In other cases, especially in insects, the ovum grows rich at the expense
of neighbouring cells, which are sacrificed to its nutritive equipment. But
it is evident enough that a cell remains a cell, however many of its neigh-
bours it may happen to absorb.
§ 5. Egg Envelopes, — The ovum starts as a naked cell, but generally
becomes fiirnbhed with ensheathing envelopes. The exact history of the
egg-membranes and sheaths is a very complex matter. Only the most
general facts can here be stated. The envelopes may be derived (a) from
the ovum itself, {b) from surrounding cells, [c) from the secretion of special
glands.
(a) Just as a protozoon often exhibits distinct outer and inner zones,
distinguished by minor physical and chemical peculiarities, so it is with the
ovum. What are called yolk or vitelline membranes are generally pro-
duced by the ovum itself. Furthermore, the outer protoplasm oAen forms
a distinct firm zone, known as the Zonapellucida, This may be traversed
by fine radiating pores establishing nutritive communication with the
exterior, and is then known as the Zona radiata, A special aperture or
micropyU is sometimes present, through which the sperm enters, or nutri-
tive supply is sustained.
{J>) The ovum, in its young stages, is very frequently seen surrounded
by a circle of small cells, which form what is called a follicle. These may
produce a membrane or a glairy investment.
[c) As the ovum ripens, and passes from the ovary into the duct, it
often becomes surrounded by gelatinous, homy, limy, and other invest-
ments. In most cases, it necessarily follows that the egg has first been
fertilised. The investments are usually referable to the activity of the
walls of the oviduct or uteHis, though sometimes there are special shell-
glands, and the like. The chitinous cases of some insect ova, the horny
mermaids' purses of many gristly fishes, the more or less limy egg-
envelopes of reptiles, the firm limy egg-shells of birds, so often stained with
pigments, afibrd good illustrations of these secondary investments. Quite
distinct are cocoons, such as those of earthworm and leech, which
surround several eggs, and are produced from the skin of the animal.
It may here be noted that the rather puzzling bodies known as ** wind-
eggSf" or niore naively as cock's eggs, and regarded by some as the
products of immature or exhausted females, are most probably not eggs at
all, hut simply masses of albumen formed in the oviduct and coated with a
shell. (Raspail, *' Bull. Soc. Zool. France," xxiii., 1898, pp. 94-97.)
§ 6. Bird^ ^g%s, — The student may be fitly directed to the
egg of the fowl, or of some other bird, for a convenient concrete
illustration of many facts. There he will see the great mass of
THE EGG-CELL OR OVUM. Ill
yolk, of two kinds, yellow and white, and on the top of this the
minute area of formative protoplasm. It was on this, as it
gradually revealed the cloudy outlines of the embryo chick,
that the Greeks looked with naive unaided eyes. Here it was
that Aldrovandus, Harvey, Malpighi, Haller, and the early
embryologists, with clear vision, saw almost as much as their
appliances would permit. It was this which, in its primitive
simplicity, impressed Wolff with the reality of epigenesis; and
it is this that the observers of to-day look down upon through
their embryoscopes, or cut sections of with their microtomes.
Then round about all is the secondary investment of "white of
egg" or albumen; round this a shell membrane, between the
two layers of which the little air-chamber is formed at one end ;
and finally, the hard but porous limy shell. Mr Irvine, of
Granton, has shown that fowls kept with access to no carbonate
of lime, but only to other salts of lime, can still form a normal
shell. This still consists of carbonate of lime, and is as firm as
usual, demonstrating, like the same investigator's experiments
on crabs, that animals possess no little power of changing one
salt of lime into another. Then, in the eggs of other birds,
the import of the seven or more pigments which produce the
marvellous variety and beauty comes into question. Sorby has
shown that they are related to the pigments of blood and bile ;
but what they exactly mean no one yet knows. Wider still,
the problem arises of how this coloration is so often pro-
tective. Or again, there is the curious fact that the size of
the egg is often much out of proportion to the size of the
bird, and the question arises as to how far this can be inter-
preted as the result of the more or less anabolic and sluggish
constitution.
§ 7. Chemtstry of the Egg. — Every one knows that the eggs of birds
form highly nutritious diet. As the egg contains nourishment for the young
bird for a considerable time, it must, like milk, contain all the essentials of
food. The results of a recent analysis of the fowl's egg may be taken as a
sample.
The germinal or formative disc consists chiefly of albuminoid bodies,
apparently of the globulin group, plus smaller quantities of lecithin and the
like. The subtle protoplasm itself, it need hardly be said, defies analysis.
In the yolk there are firm fats (tripalmitin, probably plus a little
stearine), and a fluid oil or glyceride. Fatly acids develop during hatching.
A relatively large quantity of lime is present, probably, for the most part,
as calcium albuminate. In the white of egg there are true albumins, also
globulins, and the quantity of peptones increases with the age of the egg.
112 THE EVOLUTION OF SEX.
During development the embryo becomes richer in mineral matters, fat,
and albumen, and the dry substance of the whole contents of the egg
diminishes considerably.
The yolk of many different kinds of ova has been analysed, and the
component substances distinguished as Ickthin (fishes), Emydin (tortoise),
and the like. More important were the discoveries of choUsterin^ vilellin^
nuciein, Ucithin^ and, in association with the latter, ncuriu. As we can-
not here enter into the ph3rsiological import of such substances, it is enough
to say that the nutritive material in ova usually consists of a mixture of com-
plex, unstable, and highly nutritive substances.
§ 8. Maiuradon of the Ovum, — When the egg-cell has
attained its mature size, a more or less enigmatical occurrence
takes place. The nucleus, hitherto generally central, moves to
the pole, alters considerably in its structure, and divides. A
minute cell, with half of the nucleus, and a small amount of
protoplasm, is given off. Not long after, the nucleus remaining
within the ovum repeats the process, and another tiny cell is
expelled. This process is known as the extrusion of the polar
globules. Of general, and probably of universal occurrence,
it has been most satisfactorily studied in invertebrate types.
There is considerable diversity as to the exact time at which
the extrusion occurs; generally, however, it precedes the
entrance of the fertilising sperm. The minute extruded cells
never have any history, though they occasionally linger for a
considerable time on the outskirts of the ovum. As an excep-
tion, they have been seen themselves to divide, and, with
equal rarity, a misguided spermatozoon has been observed to
penetrate them. Usually, however, they simply dwindle away.
The remaining female nucleus of the ovum is now ready to
unite with the male nucleus of the spermatozoon. At this
point, awaiting the essential moment of fertilisation, we shall
for the present leave it
Weismann, assisted by C. Ischikawa, has demonstrated an
exceedingly interesting fact in regard to polar globule extrusion
in parthenogenetic ova. Instead of the two polar globules
which are usually extruded, parthenogenetic ova were shown to
form only one. This was demonstrated in a variety of cases, —
in water-fleas (daphnids and ostracods) and rotifers, — and is
believed to be a general fact. Blochmann, who has been suc-
cessful in demonstrating polar globules in several orders of
insects, has also observed that in the parthenogenetic ova of the
plant-louse or aphis, only one polar globule is formed, while
in other non-parthenogenetic aphid eggs, which only develop
THE EGG-CELL OR OVUM. tI3
af^er fertilisation, two occur as usual. To these facts we must
afterwards recur in connection with parthenogenesis.
What must be emphasised, however, is this : — the nuclei of
the mature ovum and spermatozoon contain half the number
of nuclear rods or chromosomes characteristic of the body-
cells of the animal in question. In their early immature stages
they contain the normal number. Therefore a reduction — a
halving — of the number must take place during the process of
maturation. The same is true in plants.
Similarly, before the nuclear fusion in two conjugating
individuals of the sun-animalcule (Aciitwphfys sol\ there is,
as Schaudinn has shown, a " reduction-division," half of each
nucleus being got rid of, and there are several other pheno-
mena in Protozoa which appear to be analogous to the matura-
tion-processes in the ova of Melazoa.
§ 9. Theories of the Polar Globules. — The polar globules appear to
have been first observed in 1848 by Fr. MUtler and Ix>v^n, but it is only
within recent years that much has been made of them. Thanks to the
masterly researches of BiUschli and Hertwig, Giard, Fol, and others, it
became possible to interpret the extrusion as a case of cell -division or
budding. More recently, \'an Beneden, whose monograph on the ovum of
the threadworm {Ascaris) will remain one of the classics m this department
of research, has raised a protest against regarding the extrusion as a
normal cell-division. The details of the process, as interpreted by him,
seemed to mark out the extrusion as something unique. Most authorities,
however, adhere to the older view, that the process is essentially one of
normal cell-division.
As to the meaning of the process, the chief opinions, only a mere out-
line of which can be given, are three, not including a number of suggestions
according to which the extrusion of the globules is a kind of " excretion "
of the ovum, or a ** rejuvenescence " of the nucleus.
(a) According to some, the egg-ccU is in a sense hermaphrodite, and
the polar-globule formation is an extrusion of the male element. Balfour
expressed his view in somewhat teleological language: — ** I would suggest
that in the formation of the polar cells, part of the constituents of the
germinal vesicle, which are re!quisite for its functions as a complete and
independent nucleus, is removed to make room for the supply of the neces-
sary parts to it again by the spermatic nucleus. ... I will venture to
add the further suggestion, that the function of forming polar cells has
been acquired by the ovum for the express purpose of preventing partheno-
genesis. To this it must now l)e pointed out, that so far as one polar
globule is concerned, extrusion does not prevent parthenogenesis. This
view seems, according to Brooks, to have been first advanced by M'Crady.
It has been most carefully elaborated by Minot. According to Minot,
**in the cells proper, both sexes are potentially present ; to produce sexual
elements the cell divides into its parts ; in the case of the egg-cell, the
male polar globules are cast off, leaving the female ovum." In partheno-
genetic ova, he supposes that enough male element is retained, since only
IT4 THE EVOLUTION OF SEX.
one polar globule appears to be formed. Van Beneden, whose opinion is
entitled to great weight, also inclines to regard the polar globules as male
extrusions.
Sabatier distinguishes, besides true polar globules, other extrusions, and
believes the eliminated parts to be male elements. His views are connected
with an elaborate theory of polarities, according to which, for instance, the
peripheral extrusions are male, while central cores (in the development of
sperms) are female residues.
(d) A very different view — morphological rather than physiological —
has been maintained by Giard (1876), Mark (18S1), Butschli, Whitman,
and others, that the polar bodies are equivalent to ova. The formation of
polar globules is an atavistic reminiscence of primitive parthenogenesis.
Just as the mother sperm-cell or spermatogonium, which corresponds in the
male to the ovum in the female, divides up into what form spermatozoa, so
the ovum retains a slight power of division. In short, the polar globules
are "abortive ova."
{c) According to Weismann, Hertwig, and others, the gist of the matter
may be expressed by the term " reducing-division." In different species
the cells of the body are characterised by the possession of a definite
numl>er of chromatin elements. Thus, in one variety of the round-worm of
the horse there are four, in man there are two hundred. In the maturation
of ovum and spermatozoon the number is reduced preparatory to fertilisa-
tion, so that the fertilised ovum contains the normal quota, and not double
that as would be the case were there no reduction. The period and the
details of the reduction seem to differ greatly, but the general fact stands
out clearly that maturation implies a reduction of the number of chromo-
somes in the ultimate germ-cells to one-half the number characteristic of
the somatic cells of the species. The reduction phenomena occur in plants
as well as in animals, but the details remain somewhat uncertain. In the
higher plants the reduced number is seen in all the cells of the sexual or
gametophyte generation, beginning with the asexual mother-s]X)re-ceIls
from which this generation arises.
THE EGG-CELL OR OVUM. XI5
SUMMARY.
1. The ovum presents all the essential features of a cell — cytoplasm,
nucleoplasm, etc.
2. The ovum often passes from an amoeboid to an encysted phase, Nvith
increase of nutrition and size.
3. The yolk is derived from the vascular fluid, or surrounding cells, or
special glands, and is present in varying quantity and disposition. If little,
it is diflfuse; if much, it is polar or central ; and the different modes of egg-
division are associated with this.
4. In some cases the ovum is surrounded by a number of nutritive cells
(composite ova), and often becomes what it is by preying upon its neigh-
bours. This hardly affects its unicellular character.
5. Egg-envelopes are produced from the ovum itself {e.j^., vitelline
membrane), or from surrounding cells (follicular sheath), or from special
glands (the outside shell).
6. Bird's egg noted as a concrete illustration of facts and problems.
7. The egg, so far as its nutritive material is concerned, includes a
mixture of complex, unstable, highly nutritive substances.
8. The maturation of the ovum is usually associated with a double cell-
division, known as the extrusion of polar globules. In parthenogenelic
ova only one occurs, with rare exceptions.
9. These polar globules have been interpreted variously: — {a) As
extrusions of male elements; or (3) as abortive ova; but {c) the whole
process implies a reduction of nuclear rods or chromosomes, preparatory to
fertilisation.
LITERATURE.
Balfour, F. IL—Op, cit.
Van Bbnbdbn, E. — Recherches sur la F^ondation. Arch, de Biologic,
IV., 1883.
Carnoy. — La Cellule, IL, 1886, etc.
Gbddbs, p. — Op, cit,
H ADDON, A. C—Op, cit,
Haecker, V. — The Reduction of the Chromosomes in the Sexual Cells.
Annals of Botany, IX., 1895, pp. 95-101.
Hensen, V,—Op, cit,
Hertwig, O.—Op, cit,
Hatchett Jackson.— Introduction to his edition of Rolleslon's Forms
of Animal Life.
M'Kendrick, J. G.— On the Modern Cell Theory, &c. Proc. Phil. Soc.
Glasgow, XIX., x888.
MiNOT, C. S. — American Naturalist, XIV., 1880.
Strasburger. — The Periodic Reduction of the Numl)er of the Chromo-
somes in the Life-history of Living Organisms. Annals of Botany,
VIII., 1894, pp. 281-316.
Il6 THE EVOLUTION OF SEX.
Thomson, J. A. — Recent Researches on Oogenesis. Quarterly Journ.
Micr. Sci., XXVI., 1886.
Art. Embryology, Chambers's Encyclopsedia.
W&ISMANN, A. — Die Continuitat des Keimplasmas. Jena, 1885.
Die licdeutung der sexuellen Fortpflanzung. Jena, 1886.
" ■ And other papers translated, "Essays on Heredity," etc. Oxford.
1889.
The derm IMnsni. London, 1S93.
Wilson, K. B.— TIjc Cell in Development and Inheritance. New York,
J 896, 371 pp., 142 Hgs.
CHAPTER IX.
The Male-Cell or Spermatozoon.
§ I. The Gefierai Contrast between Ovum and Spermatozoon,
— Just as the ovum, large, well nourished, and passive, is as
such a cellular expression of female characteristics, so the
smaller size, less nutritive habit, and predominant activities of
the spermatozoa illustrate the qualities of maleness. As the
ovum is usually one of the largest, the sperm is one of the
smallest of cells, sometimes only xWinnr^^ ^^ ^^^ size of the
ovum, which is often microscopic. The yolk or food-capital,
and encysting membranes, which are often so prominent in the
former, are as conspicuously absent in the latter. The con-
trast, though less accented, is still quite discernible in plants.
In fact, the two kinds of cells are just as widely opposed in
their general features, as they are fundamentally complemen-
tary in their history. Before this opposition and complemen-
tariness can be fully understood, however, we must briefly sum
up the characters and history of the male elements.
§ 2, History of Discovery. — In 1677, one of Leeuwenhoek's students,
Hamm by name, called his master's attention to the minute elements
actively moving in the male fluid. Leeuwenhoek, who some years pre-
viously for the first time observed what we now know sis unicellular
organisms, was at once impressed by the import of the marvellously active
male units. Almost too much impressed, in fact, for he interpreted them
as minute preformed germs, which only required to be nourished by the
ovum to unfold into embr>'os. Thus the unfortunate aberration, already
noted as the doctrine of the animalculists, had its origin. For long no
progress whatever was made; some naturalists, like Vail isneri, depreciating
the import of the sperms altogether, and regarding them as worms which
hindered the coagulation of the seminal fluid ; others going to the opposite
extreme, and regarding them as nests of germs. Thus Haller at first
considered them to l)e what Leeuwenhoek had suggested, but afterwards
admitted ihem merely as naiivi hospites seminis. In 1835, even Von Baer
was inclined to interpret them as minute parasites peculiar to the male
fluid; and if the curious student will turn up the article Entozoa in Todd's
**Cyclop£edia of Anatomy and Physiology," of about the same dale, he
will find that the vetetan 0\^'en includes the spermatozoa under that strange
Il8 THE EVOLUTION OF SEX.
heading. The very name spermatozoon recalls the view which so long
prevailed.
In 1837, K. Wagner emphasised their constancy in all the sexually
mature males which he examined, and their absence in infertile male
hybrids. Von Siehold demonstrated their presence in many of the lower
animals; and lastly, in 1841, KoUiker made one of his many important
contributions to biology, in proving that the sperms had a cellular origin in
the testes.
§ 3. Structure of the Sperm, — The sperm, then, is a cell.
Though some, such as Kolliker, have inclined to regard it
rather as a nucleus, its truly cellular character has been proved
beyond dispute. As in the ovum, there is cell-substance and
nucleus, with this marked difference, that the cell-substance is
generally reduced to a minimum.
The sperm is almost always, moreover, a cell of a very
definite type or phase. It is like one of the highly motile
I
J
*' S]>criuatic Animalculi " of the Rabbit and the Dog.
— From Buifon, after Leeuwenhoek.
Protozoa, like a flagellate infusorian. Usually it consists of a
minute "head," consisting almost entirely of nucleus, and of a
long contractile tail, which, working behind like a screw, propels
the essential **head" through the water or along the ducts.
Between the head and the tail there is an important middle
portion, which many observers agree in regarding as the bearer
of the centrosome — a now well-known component of a typical
animal cell.
Occasionally there is a departure from the usual flagellate
type. Thus in the threadworm Ascaris^ the sperm has a blunt
pear-shaped form, and exhibits slight amoeboid movements.
In some crustaceans and other arthropods, the cell is even more
quiescent, and may exhibit curious forms, such as that figured
for the crayfish. The relatively dormant activity may however
wake up, and the sperm exhibit active amoeboid movements.
Zacharias has made some interesting experiments, showing the
tHE 1>IAL£-CELL Oft SPfeRMAtOZOO^i.
11$
Miodifiability of sperms under reagents; thus, in a little crus-
tacean {Polyphemus pediculus)^ he first caused the cylindrical
sperm to form amoeboid processes, and afterwards to replace
these by what were to all intents and purposes cilia. This is
entirely congruent with other experiments and observations on
the i>assage of cells from one phase of the cell-cycle to another.
F. Silvestri records the interesting case of the millipede
Pachyiulus communis in which the spermatozoon is immobile
and cap-shaped, and is drawn into the ovum by a pseudo-
podium emitted by the latter through the micropyle. In other
words, the ovum here plays the active rdle, and the spermato-
zoon is passive. ("Atti Ace Lincei (Rend)," vii., 1898, pp
129-133^ 5 figs)
Spermatozoa of crayfish (<«), lobster (Jf\ crab (c), ascarid {d)y
water-flea — Moina («r), man (/), ray (^), rat {h\ guinea-
P>S (Ot A beetle — immature stage (it), sponge (/). (Not
drawn to scale.)
The progress of microscopic teclinique has demonstrated many com-
plexities in the sperm as well as in the ovum. For a discussion of some of
the more important of these, the reader is referred to the ** Encyclopaedia
Britannica," article Refrodttciion. A few points only need be noticed here.
Thus most spermatozoa exhibit not only a head (almost wholly from the
nucleus of the mother-cell) and a mobile tail (from the substance of the
mother-cell), but a median portion connecting these. The tail is not
unfrequently, as in salamander and man, furnished with a very delicate un-
dulating or vibratile band, and often shows, as in birds, an axial filament,
which like many other contractile structures is distinctly Bbrillated. In a
few cases, as in the threadworm, the sperm is not left without any nutritive
capital, but furniiihed with this in the form of a cap, which falls off before
the essential moment of fertilisation arrives. It is very generally admitted
120 THE LVOLUTiON OK SL5i.
that the head consists almost wholly of chromatin, and that the tail is
mainly cytoplasmic, i.e.^ formed of cell-sulistance. The middle part con-
necting head and tail is believed by many to be formed by the centrosomes,
which play some part in the division of the egg. In the non-flowering
plants the male-cells or antherozoids often Ix^ar a close resemblance lo those
of animals; in the flowering plants the male-elements are the reproductive
nuclei which issue from the pollen-tube; but the discoveries of Iliras^ and
Ikeno have shown that even in flowering plants, namely in Cycads, there
are motile spermatozoa. It is of interest to notice that a dimorphism of
spermatozoa has been recorded in various cases, e,g.^ by Auerhach for
water- beetles, and even in man by Bardeleben. As has been already
noted, there is sometimes, as in Rotifers, a dimorphism of ova, and it is
probable that we have here again an illustration of the fundamental varia-
tional alternatives, — between relatively katabolic and relatively anabolic
phases. Apart from this dimorphism, it should be noted that in the testis
of many higher animals, there seems to be, as in the ovary, a division of
labour between germ-cells proper and nutritive cells auxiliary to these.
§ 4. Physiology of the Spermatozoon, — A few facts in regard
to the physiology of the sperm demand notice, (a) It is
specialised as a highly active cell; its minimal size, the usual
absence of any encumbering nutritive material, the contractility
of the tail, and the general shape, all fit it for characteristic
mobility. More than one histologist has likened it to a free
muscle-cell, or to a flagellate monad, (b) Furthermore, the
sperm has very considerable power of persistent vitality.
Not only does it often remain long unexpelled in the male
animal, without losing its functions, but it may retain its fertil-
ising power after remaining for weeks, or even months, in the
female organism. In the earthworm, the spermatozoa pass
from one worm to another, not directly to the ova nor to
female ducts, but to be stored up in special reservoirs or
spermathecse. So it is with many animals. The spermatozoa
received by the queen bee during her single impregnation, are
for a considerable period — even for three years— used in fertil-
ising successive sets of worker and queen ova. Quite unique,
however, is the case of one of Sir John Lubbock's queen ants,
which laid fertile eggs thirteen years after the last sexual union
with a male. The spermatozoa had apparently persisted all
that time. Hensen cites the facts that a hen will lay fertilised
eggs eighteen days after the removal of the cock, and that in
bats, spermatozoa may remain alive a whole winter in the
uterus of the female. In most European bats, indeed, sexual
union occurs in autumn, but the sperms are simply stored in
the uterus, for ovulation and fertilisation do not take place till
THE MALKCELL or SPERMATOZOON.
121
spring. In exceptional cases, especially in young forms which
were not mature in autumn, pairing occurs in spring. An
exactly parallel condition is known in some snakes. Thus
Rollinat notes in regard to Tropidonotus viperinus that mature
females are inseminated in the autumn previous to the egg-
laying (in June or July), but in females laying for the first time,
copulation probably occurs in early spring ("Bull. Soc. Zool.
France," xxiii., 1898, pp. 59-63). ij) Remarkable too, and again
suggestive of monads, is the power the sperms have of
resisting great deviations from the normal temperature. The
presence of acids has usually a paralysing influence, but alka-
line solutions have, on the whole, the opposite result.
n
Diagram of the Development of Spermatozoa (upper line), of the Maturation and
Fertilisation of the Ovum (lower lincX
a, an amoeboid sex-cell ; A, ovum, with germinal vesicle, n ; B, ovum extnidine first
polar body, / * ; C, extrusion of second polar body, /', nucleus m*, now reduced.
I, a mother-sperro-cell, dividing (2, 3) into immature and mature spermatozoa (x/.).
D, the entrance of a spermatozoon ; £, the male and female nuclei sp. n and n* approach
one another.
§ 5' Origin of the Sperms. — For the last twenty years the
development of spermatozoa has been the subject of almost
continuous research and controversy, and the all too-abundant
nomenclature affords a suggestive index to the confusion out
of which the subject is now emerging. In a general way, the
process is simply that of the varied segmentation of a mother-
sperm-cell, and the occurrence of a series of preparatory stages
before the sperm is finally matured. In detail, however, there
are many variations, and these are described in a maze of often
tautologous and ambiguous terms, such as spermatogonium,
122
THE EVOLUTION OF SEX.
spermatoblast, spermatospore, spermatogemma, sperniatomerr,
spermosphere, and a dozen more.
One of the most defensible set of terms is that used by Voigt after
Semper, and also by Von la Valette St George. The sperm or spermatozoon
is differentiated from an immature cell or spermatide, this is modified from
or descended from a spermatocyte, the spermatocytes result from the
division of the mother-sperm-cell or spermatogonium, and this finally is a
modified form or a descendant of the primitive sex-cell or male ovule.
B'
C
D'
Comparison of Spermatogenesis and Ovum Segmentation.
Explanation. — The first line, A-E, exhibits types of ovum segmentation: — A, regular
morula; B, unequal segmentation, r.^., m some Molluscs; C, centrolecithal or
peripheral type, e^.^ in a shrimp Peneus; D, discoidal segmentation; £, the same,
with the cells less markedly defined off from the yolk.
In the next two lines various types of spermatogenesis are collated with the above
to illustrate the parallelism : — A' and A"^ morula t^'pe, as in Sponge, Turbellarian,
Spider, &c. ; B' and B", where the division is unequal, and one large nutritive cell is
seen (Plagiostome fishes, after Von la Valette St George); C and C'^ aAer Blomfield,
Jensen, &c., showing central cytophoral or blastophoral nutritive portion; IK and D",
sperm-blastoderm, with a few formative cells on laree nutritive blastophore, after
Gilson, &c. ; £' and E", the same, with the sperm cells less definitely separated off,
after Von Ebner and his followers.
Difficulties become thick, however, when we inquire into the division of
the mother-sperm-cell or spermatogonium, and it is here that the observa-
tions of recognised authorities so much disagree. Accepting the results of
competent observers, we have elsewhere endeavoured to rationalise and
unify the conflicting observations, by comparing the different modes of
spermatogenesis with the difierent forms of ovum-segmentation. It has
THE MALE-CELL OR SPERMATOZOON. 12^
been already incidentally noticed that the egg-cell may divide wholly and
equally, or unequally, or only very partially, or round a central core.
Just in the same way the mother-sperm-cells may divide into a uniform
ball of cells, or only at one pole, or only at the periphery round a central
residue. Balfour and others had hinted at this comparison in the use of
terms like sperm -morula; and Hermann had also concluded, **that the
division of the male ovule into a series of generations of daughter-cells, is
a phenomenon comparable to that exhibited by the ovum in the formation
of the blastoderm. ... It seems then more important to determine
exactly the mechanism of division, than to give a particular name to each
stage of segmentation."
Although this interpretation of spermatogenesis by collating it with
ovum s^mentation appears to Minot ** a fanciful comparison," in favour
of which he is *' unable to recognise any evidence," neither the initial
homology between the mother-sperm-cell and ovum with which we start,
nor the striking parallelism between the modes of division of these
homologues, seem thereby even disputed, much less shaken. The widely
different conditions in which these two processes occur, and their very
different meaning to the organism, are of course obvious.
§ 6. Further Comparison of Ovutn and Sperm, — It is often said that
the sperm is the male cell which corresponds to the ovum. This is only
true in a certain sense. In function the two elements are indeed, in a
general way, of equal rank, and are obviously complementary. But even
in this respect, the two elements, which unite in equal proportions in the
essential act of fertilisation, are not exactly sperm and ovum, but {a) the
head or nucleus of the sperm and [b) the female nucleus doubly reduced by
the extrusion of two polar globules. The accurate structural resemblance
or homology seems to be between oogonium or primitive female germ-cell
and spermatogonium or primitive male germ-cell, or between the immature
ovarian ovum or oocyte and the spermatocyte.
The parallelism between the reduction of the number of chromosomes
in the maturation of the ovum and the similar reduction in the course of
spermatogenesis is of great interest and importance. Unfortunately the
results of different observers and in regard to different organisms remain
perplexingly discrepant. Professor £. B. Wilson gives the following state-
ment with special reference to the case of Ascaris megalocepkalay the
threadworm of the horse : —
" Like the ova, the spermatozoa are descended from primordial germ-
cells which by mitotic division give rise to the spemta/ogonia from which
the spermatozoa are ultimately formed. Like the oogonia, the spermato-
gonia continue for a time to divide with the usual (somatic) number of
chromosomes, z.^., four in Ascaris megalocephala bivcUetis, Ceasing for a
time to divide, they now enlarge considerably to form spcnnatocytcs^ each
of which is morphologically equivalent to an unripe ovarian ovum, or
oocyte. Each spermatocyte finally divides twice in rapid succession, giving
rise first to two daughter-spermatocytes and then to four spermatids^ each
of which is directly converted into a single spermatozoon. The history of
the chromatin in these two divisions is exactly parallel to that in the for-
mation of the polar bodies. . . . From each spermatocyte arise four
spermatozoa, and each sperm-nucleus receives half the usual number of
single chromosomes. The parallel with the egg-reduction is complete."
87. Chemistry of the .^/-///.—Comparatively little has been done in
12 1 THE EVOLUTION OF SEX.
regard to ihe chemislry of the male elements in different animals. The
most important observations are those of Mieschcr, on the milt of salmon.
His analysis demonstrated the presence of lecithin, fat, and cholesterin, —
also component parts of the ovum. Besides these, after the heads of the
spermatozoa have been formed, Miescher detected the abundant presence
of a substance which he called protamine, a compound of nucleic acid.
Albuminoid material, and products of decomposition, such as sarkin and
guanin, were demonstrated, according to Hensen, by Picard.
More recently, Kossel has shown that the protamin in the salmon's
sperms and the analogous sturin in those of the sturgeon, act as basic
substances forming a salt-like combination with the nuclein substances.
("SB. K. Preuss-Akad. Berlin," 1896, pp. 303-308.)
It is remarkable, however, as Dr T. Gregor Brodie points out
('* Science Progress," vii., 1898, pp. 131-149), that the spermatozoa
consist of sulistanccs which, relatively to proteids, are of simple con-
stitution. *' If it be true that hereditary characteristics are transmitted by
the chromatin of the reproductive cells, we should have expected a most
complex chemical structure for these parts; and it therefore becomes the
more striking to note that the most complex protamine as yet obtained,
arbacin, is from an animal low in the scale {Arbacia^ a sea-urchin), and
that in the higher vertebrates examined no protamine is present at all."
Zacharias has made a micro-chemical comparison of the male and
female elements in Characese, mosses, ferns, phanerogams, and amphibians.
He finds that the male cells are distinguished by iheir small or absent
nucleoli, and by their rich content of nuclein; while the female elements
exhibited a poverty of nuclein, an abundance of albumen, and one or more
nucleoli, more or less large in proportion. The male cells have, in
relation to their protoplasm, a larger nuclear mass than the female
elements.
It is interesting to notice that an analysis of two different kinds of
pollen showed a great analogy of composition between these male repro-
ductivc cells and those of the salmon and ox.
THE MALE-CELL OR SPERMATOZOON 1 25
SUMMARY.
1. The contrast between the elements is that between the sexes. The
large, passive, highly- nourished, relatively anabolic ovum; the small,
active, relatively katabolic sperm.
2. Hamm's discovery, 1677 ; Leeuwenhoek's interpretation ; the school
of animalculists ; Kolliker's demonstration of the cellular origin of the
sperm, 1S41.
3. Structure of the sperm, — nuclear "head" of chromatin, protoplasmic
"tail," middle portion. The sperm comparable to a monad or flagellate
infusorian, only with less cell-substance. Its occasional degradation into
the amoeboid phase.
4. Physiology of the sperm ; its locomotor energy at a maximum, but
yet great power of endurance, like a monad or bacillus.
5. Origin of sperms from the division of a molher-sperm-cell homo-
logous with the ovum. The diflTerent modes of "spermatogenesis " may be
collated with the different modes of ovum-segmentation.
6. The occurrence in sperm-development of phenomena comparable
both structurally and functionally with polar-globule formation.
7. Chemistry of the sperm ; resemblance between pollen and sperma-
tozoa.
LITERATURE.
Ballowitz, E. — Structure of Spermatozoon. Archiv. Mikr. Anat.,
XXXII., 1888, pp. 401-473, 5 pis.
Gbddes, p., and Thomson, J. A. — History and Theory of Spermato-
genesis. Proc. Roy. Soc. Edin., 1886, pp. 803-823, i pi. See also
Zoological Record from 18S6.
HiRAsA, S.— Journ. CoH. Sci. Tokyo, XII., 1898, pp. 102-149, 3 P^s .
2 figs.
Ikeno, S.— Jahrb, wiss. Bot., XXXII., 1898, pp. 557-602, 3 pis., 2 f)g«.
Journ. Coll. Sci. Tokyo, XII., 1898, pp. 151-214, 8 pi.?.
Wilson, E. B. — The Cell in Development and Inheritance, 1896; new
ed., 1900.
CHAPTER X.
Theory of Sex — ns Nature and Origin.
Having got so far in our analysis, and before passing to the
study of the processes of reproduction, we must add up the
results in a general theory of the nature and origin of sex.
After this has been done, we shall be in a better position to
deal, in Book III., with fertilisation, parthenogenesis, and the
like. The number of speculations as to the nature of sex has
been well-nigh doubled since Drelincourt, in the last century,
brought together two hundred and sixty-two "groundless
hypotheses," and since Blumenbach quaintly remarked that
nothing was more certain than that Drelincourt's own theory
formed the two hundred and sixty-third. Subsequent in-
vestigators have, of course, long ago added Blumenbach's
" Bildungstrieb " to the list ; nor is it claimed that the
generalisation we have in our turn offered has yet received
"final form," if that phrase indeed be ever permissible in an
evolving science, except when applied to what is altogether
extinct. This much, however, is distinctly maintained, that
future developments of the theory of sex can only differ in
degree, not in kind, from that here suggested, inasmuch as the
present theory is, for the first time, an expression of the facts in
terms which are agreed to be fundamental in biology, those of
the anabolism and katabolism of protoplasm.
§ I. Suggested Theories, — According to Rolph, — a fresh and
ingenious thinker, removed before attaining his mature strength,
— " the less nutritive, and therefore smaller, hungrier, and more
mobile organism [cells, he is speaking of] we call the male;
the more nutritive, and usually more quiescent organism is
the femiile." He goes on vividly to suggest why "the small
starving male cell seeks out the large well-nourished female ce)l
for the purposes of conjugation, to which the latter, the larger
and better nourished it is, has on its own motive less
inclination."
THEORY OF SEX — ITS NATURE AND ORIGIN. I27
Minot, in his " theory of genoblasts/' or sexual elements,
ventures little further than regarding male and female as
derivatives of primitive hermaphroditism in two opposite
directions. "As evolution continued, hermaphroditism was
replaced by a new differentiation, in consequence of which the
individuals of a species were — some capable of producing ova
only, others of producing spermatozoa only. Individuals of
the former kind we call females, of the latter males, and they
are said to have sex." " At present all we can say is, we do
not know why or how sexual individuals are produced." In
regard to the sex-elements, we have already noticed his opinion
that they are at first "hermaphroditic or asexual," and that
both differentiate by the extrusion or separation of the con-
tradictory elements, the ovum getting rid of male polar globules,
the sperms leaving behind a female mother-cell-remnant.
Brooks has emphasised rather a different aspect of the
question. "A division of physiological labour has arisen
during the evolution of life, the functions of the reproductive
elements have become specialised in different directions."
" The male cell became adapted for storing up gemmules, and,
at the same time, gradually lost its unnecessary and useless
power to transmit hereditary characteristics." "The males
are, as a rule, more variable than the females; the male leads,
and the female follows, in the evolution of new races." Brooks
does not exactly attack the problem of the nature and origin
of sex, but his hypothesis of the greater variability of males is
of interest.
To others, again, it seems sufficient to interpret sexual
dimorphism as an adaptation evolved in the course of natural
selection from variations of which no rationale can be given.
As to the adaptive character of sex there can be no doubt,
our problem is whether the variations which gave rise to the
dimorphism may not be interpreted in terms of the two main
alternatives of protoplasmic metabolism.
§ 2. Nature of Sex as seen in the Sex- Elements — The Cell
Cycle, — As ova and sperms are the characteristic products of
female and male organisms, it is reasonable that an interpreta-
tion of sex should start at this level. Here, assuredly, the
difference between male and female has its fundamental and
most concentrated expression. For the bodies, after all, as
Weisniann has so clearly emphasised, are but appendages to
this immortal chain of sex-cells.
128
THE EVOLUTION OF SEX.
We have already pointed out that the sex-cells are more or
less on a level with the Protozoa. If we only knew, they
probably differ widely from them in those intricacies of nuclear
structure of which we only see the surface; yet as single cells
the sex-cells arc comparable with the Protozoa. For ihe
moment, let us study those simplest organisms. When we
consider an extended series of unicellular forms, amoebae,
foraminifers, sun-animalcules, Infusorians, gregarines, and some
of the simplest algae as well, we gradually begin to group these
in the mind under three divisions. First there are highly active
cells,— infusorians of all sorts; at the opposite extreme there
The divergence of male and female cells from
piimitive amoeboid indifference.
are qu'escent forms, in which the life seems to sleep, and loco-
motion is almost absent, — the gregarines, and some unicellular
algae; and between these there are forms which in a via media
have effected a sort of compromise between activity and pas-
sivity, which are without the cilia of the one or the relative
stagnancy of the other, but possess outflowings of their living
substance, — the familiar amoeboid processes. We may thus
reach, almost by inspection, a rough and ready classification
of the Protozoa, into active, passive, and amoeboid cells, — a
classification which, under varying titles, is more or less dis-
tinctly recognised by ail the authorities on the subject.
THEORY OF SEX— ITS NATURE AND ORIGIN. lag
Cut if wfC go further than casual inspection, and study the
life-history of some of the very simplest forms, such as some
of the primitive Proteoniyxa and Myxomycetts, and follow, for
instance, Haeckel's account of the life-cycle in Protomyxa, we
soon gain new light on our classification. For in these hfe-
The evil of Ptvlimnjii buntiisE, ifac ILigeltale youns %a.te» bccomine ioia:bi>id, and
evennully unitini ui ■ <>^i»siu amoeboid mui, or ''^plaunodium."— Prom HnKkcl.
histories we lind the cells now encysted, now active lashed spores,
and again sinking down into the compromise of equilibrium
J30
THE EVOLUTION OF SEX.
effected by amoebse. We are now in a position to recognise
that the chapters in the life history of the simplest forms are,
as it were, prophecies of each of the three groups. In other
words, the most primitive organisms passed through a cycle
of three phases, one of which is accented by each of the three
main groups of the Protozoa. And while each main group is
characterised by one dominant phase of cell-life, — encysted,
amoeboid, or flagellate, — there are often transient hints of
Diagram of the Cell-cycle,— of encysted (K), ciliated (C), and amceboid
(A) phases. I., II., III., in Protozoa; IV., ovum and sperm
of fern prothallus: V., encysted, ciliated, and amoeboid
animal cells; Vl., ciliated animal cell pathologically
becoming amoeboid; VII., typical and amceboid sperm;
VIII., amoeboid and encysted ovum. — From Geddes.
other phases. Thus an infusorian has its encysted chapter,
a gregarine its amoeboid stage, and a rhizopod may begin as
a mobile ciliated spore ; for each group, while accenting one
phase of the cycle, retains embryonic reminiscences of the
others.
We become more convinced that the triple division really
means much when we pass from the Protozoa to the cells which
compose higher animals. There we find active ciliated cells
THEORY OF SEX — ITS NATURE AND ORIGIN. I3I
in most of the classes, from the ciliated chambers which lash
the water through a sponge, to the cells lining the air passages
in man; passive encysted cells are illustrated in some forms of
connective, fatty, and skeletal tissue; while the white blood
corpuscles are obviously comparable to amoebae. Extended
observation here also shows us the cells passing from one phase
to another. Our rough classification of the Protozoa is verified
in the histology of higher animals, and reappears in the study
of their diseases. We are thus at length in a position to say,
that however these three phases weVe brought about, the forms
characteristic of them are of such wide occurrence through
nature as to justify our restatement of the familiar cell theory
in terms of a larger conception, that of the cell-cycle. That is
to say, from the conception of the cell as a corpuscle of living
protoplasm, amoeboid, encysted, or filiated, as the case might
be, we come to regard these forms as the predominant phases
of a cycle, — primeval, certainly, in the history of the organic
world, and largely so even in the individual cell.
A final corroboration of the cell-cycle, and at the same time
a rationale of it, is obtainable on physiological lines, when we
inquire into the protoplasmic processes which lie behind the
changes in the form and habit. We have already spoken of
the modem physiologist's conception of living matter, or proto-
plasm, as an exceedingly complex and unstable substance or
mixture of substances, undergoing continual chemical change
or metabolism. On the one hand, there is a process of con-
struction; the income of nutritive material, at first more or
less simple, is worked up by a series of chemical changes till it
reaches a climax of complexity and instability. These upbuild-
ing, constructive, synthetic processes are summed up in the
term anabolism. But, on the other hand, there is a process
of disruption; the complex material breaks down into more
and more stable compounds, and finally into waste products.
This disruptive, descending series of chemical changes is known
as katabolism. Both constructive and disruptive changes occur
in manifold series. The same summit may be gained or left
by many difi*erent paths, but at the same time there is, as it
were, a distinct watershed, — any change in the cell must tend
to throw the preponderance towards one side or the other. In
a certain sense too the processes of income and expenditure
must balance, but only to the usual extent, that expenditure
must not altogether outrun income, else the cell's capital of
13a THE EVOLUTION OF SEX.
living matter will be lost, — a fate which is often not successfully
avoided The disruptive, or katabolic, or energy-expending
set of changes may be obviously greater in one cell than in
another, in proportion to the constructive or anabolic processes.
Then, we may shortly say that the one cell is relatively more
katabolic than the other, or vice versd on the opposite supposi-
tion. Just as our expenditure and income should balance at
the year's end, but may vastly outstrip each other at particular
times, so it is with the cells of the body. Income may for a
time preponderate, and we increase in wealth, or similarly, in
weight, or in anabolism. Conversely, expenditure may pre-
dominate, and business may be prosecuted at a loss ; just as
we may live on for a while with loss of weight, or with excessive
wear and tear. This losing game of life is what we call a
katabolic habit, tendency, or diathesis; the converse gaining
one being the anabolic habit, tendency, or diathesis. The
words anabolic and katabolic are, of course, unfamiliar, and
undeniably ugly. Habit and temperament have very vague
associations, and tendency sounds metaphysical; diathesis,
again, seems no better than the medical equivalent of this.
These things the reader must naturally feel ; yet the medical
man has some definite concrete meaning in his. mind when he
speaks of gouty or neurotic diathesis, of bilious habit, strumous
tendency, or the like. There is more than metaphysical vague-
ness in his phraseology, and in ours.
We are now in a position profitably to return to the Pro-
tozoa, to the phases of cell-life, and to the sex-elements. After
what we have just said, it is evident that there are but three
main physiological possibilities, — preponderant anabolism, or
predominant katabolism, or an approximate (i>., oscillating)
equilibrium between these tendencies. A growing surplus of
income, a lavish expenditure of energy, or a compromise in
which the cell lives neither far below nor quite up to its
income. Great passivity, great activity, or a safe average
between these ; conservative accumulation, spendthrift liberal-
ism, and a compromise between these. In many different
ways, more or less metaphorical, we may express the plain and
indubitable facts of anabolism and katabolism within the living
matter. The student may think of the processes, with some
degree of accuracy, under the metaphor of a ceaseless fountain,
which, while remaining approximately constant, is the expres-
sion of continual ascent and descent of drops. The protoplasm
THEORY OF SEX — ITS NATURE AND ORIGIN. 133
itself must often be in as ceaseless change as the apex of the
jet.
In active, motile, ciliated, or flagellate cells, whether they be
constant forms or only phases, there is relatively predominant
katabolism, — predominant when compared with the life ex-
penditure of a passive, quiescent, enclosed, or encysted cell.
In amoeboid organisms these extremes are avoided; there is
certainly great amplitude of variation still, but neither anabolism
nor katabolism gains the ascendant in any marked degree.
Suppose, then, in such an amoeboid cell, a continued
surplus of anabolism over katabolism, the result is necessarily
a growth in size, a reduction of kinetic energy and movement,
an increase in potential energy and reserve food- material.
Irregularities will tend to disappear, surface-tension too may
aid, and the cell acquires a spheroidal form. The result —
surely intelligible enough — is a large and quiescent ovum.
It will be remembered that young ova are very frequently
amoeboid; that with a copious nutrition this disappears in
varying degrees of encystment; that ensheathing envelopes
arising from the ovum, sweated oif like cysts round Protozoa,
are exceedingly common ; and that ova are the largest of all
animal cells.
Starting once more from an amoeboid cell, if katabolism
comes to be more and more marked, the increasing liberation
of kinetic energy thus implied must find its outward expression
in increased activity of movement and in diminished size ; the
more active cell becomes modified in form, in adaptation to
passage through its fluid environment, and the natural result is
a flagellate sperm.
In short, then, the respective morphological characters of
the sex-cells, female and male, find the same physiological
rationale as do the large passive encysted and smaller active
ciliated phases of the cell-cycle in general, and are alike the
outcome and expression of predominant anabolism and kata-
bolism respectively. Here again we reach the same formula
as before ; or, more cumbrously in words — the functions are
either self- maintaining or species- maintaining, individual or
reproductive ; the former are divided into anabolic and kata-
bolic, the latter into male and female. But the second set of
products and processes, so far from being unrelated to the
other, as is commonly supposed, are in complete parallelism.
Femaleness is relative anabolic preponderance in reproduction,
134
THE EVOLUTION OF SEX.
hence the ovum has necessarily the general character which
this "diathesis" produces in non- reproductive cells; and,
similarly, relative katabolic preponderance stamps its character
of active energy upon the spermatozoon as naturally as upon the
ciliated cell or the monad.
Here and throughout it must be remembered that we are
dealing with relative preponderance of anabolism or kata-
bolism, — with ratios in metabolism. The more exact expres-
A
sion of our point is that the ratio vr in the female is greater
than the ratio ^t in the male.
Diagram showing the divergence of ovum and spermatozoon
from an undifferentiated amoeboid type of celU
Rolph's characterisation of the male cells as hungry and starving (kata-
bolic) has been experimentally confirmed by their powerful attraction to
highly nutritive fluids, and is every day illustrated in their persistent
attraction to the ova. Plainer has suggested, in the intimately herma-
phrodite gland of the snail, that the external cells which form the ova are
better nourished than the central cells which divide into sperms. Just as
an infusorian in dearth of food is known in some cases to divide into many
small individuals, so the raother-sperm-cell is perhaps the seat of similar
katabolic necessities. The long persistence of vitality seems at first sight a
difficulty, if the sperms are highly katabolic cells. It must be noticed,
however, (a) That there is often only retention, not continuance of activity,
e.g. , when the sperms lie closely packed in the special storing reservoirs ;
{b) That the secretions of the female ducts probably afford some nutriment
to the sperms, which expose an exceptionally large surface in proportion to
their mass; and {c) That to a certain extent we may think of them as
protoplasmic explosives, which may remain long inert, but on the presence
THEORY OF SEX— if S NAT*URfe AND ORIGIN. ijg
of the required stimulus are able to start again into extraordinary activity.
That they have, like the exhausted infusortans in Maupas's experiment,
reached the limit of their dividing power, is also evident. We might
refer, for instance, to Von Bardeleben's observations of futile attempts at
division (cytokinesis without karyokinesis) in the final stages of spermato-
genesis in man ("Jena. Zeitschr. Nat.," xxxi., 1898, pp. 475-520, 3 pis.,
5 figs.)-
Professor Giard has discussed the question of the possible partheno-
genesis of the male elements or microgametes ('*Comptes Rendus Soc
Biol. Paris," 1899). He suggests that the phenomena observed by Delage,
and termed ** merogonic," where a non-nucleated ovum-fragment, fertilised
by a spermatozoon, proceeds to develop normally, may be regarded as
cases where the microgamete or sperm-cell develops parthenogenelically.
Siedlecki has observed the parthenogenetic development of the micro-
gamete of the Sporozoon Adelea ovaia; the same, as Klebs and others
have shown, may occur in the lower plants.
§ 3. The Pfobiem of the Origin of Sex, — We must now
return once more to the standpoint of the empirical naturalist,
and set out towards the interpretation of sex from a different
side, that of its origin.
It has often been raised as a reproach against the now
fortunately dominant school of evolutionist naturalists, that
they could give no account of the origin of sex ; and it must
be admitted that there have not been many vigorous attempts
to tackle the problem. Apart from the simple fact, that evo-
lutionist biology is still young, there are three reasons for the
comparative silence in regard to the problem of sex.
(i.) The first of these is the prevalent opinion, that when
one has explained the utility or advantage of a fact, one has
sufficiently accounted for it. This opinion rests on an accept-
ance of the selection theory, and on a willingness to cover all
questions of origin with a frank '* ignoramus," or with the
vague assumption of an indefinite variability which affords
selection sufficient material to work upon. Our attempt is to
push the postulate of indefinite variability as far back as
possible, and to suggest a physiological reason why the varia-
tions which have led to sex-dimorphism should have taken
the lines which they presumably did. It is evident that a pre-
occupation with the ulterior benefits of the existence of sex
may somewhat obscure the question of how male and female
have in reality come to be.
(2.) A second reason for the comparative silence may be
found in the fact that the problem remains insoluble until it is
analysed into its component problems. The question of the
13(5 IHE EVOLUTION 6F SEX.
origin of sex to a mind unprepared for the consideration of
such a problem, suggests quite a number of difficulties, — What
is the import and origin of sexual reproduction (the setting
apart of special cells) ? what is the meaning and beginning of
fertilisation (the interdependence and union of sex-cells)? what
is the reason of the individual, male or female, sex in any one
case (the determination of sex) ? and lastly, what is the nature
and origin of the difiFerence between male and female?— the
question at present under discussion. For purposes of analysis,
these questions must be kept distinct.
(3.) A third reason why the problem of the origin of male
and female has been so much shirked, why naturalists have
beaten so much about the bush in seeking to solve it, is that in
ordinary life, for various reasons, mainly false, it is customary
to mark off the reproductive and sexual functions as facts
altogether per se. Modesty defeats itself in pruriency, and
good taste runs to the extreme of putting a premium upon
ignorance. Now this reflects itself in biology. Reproduction
and sex have been fenced off as facts by themselves; they
have been disassociated from the general physiology of the
individual and the species. Hence the origin of sex has been
involved in special mystery and difficulty, because it has not
been recognised that the variation which first gave rise to
the difference between male and female, must have been a
variation only accenting in degree what might be traced
universally.
§ 4. Nafure of Sex as seen in Us Origin among Plants, —
In tracing the origin of sex, we would wish to guard against
any impression of having consciously or unconsciously arranged
our facts in the light of the theory we hold. Hence we prefer
lo follow some accessible account, taken essentially from the
morphological point of view. We shall follow Professor Vines
in his article "Reproduction— Vegetable," \n\}ci^ Encydopcedia
Briiannica^ at each stage, however, endeavouring to interpret
the facts, physiologically, in the light of protoplasmic pro-
cesses.
(i.) The simple alga, Protocoaus^ common on tree-stems, in pools,
wells, and the like — reproduces itself in a simple fashion. Tiie cell divides
into a number of equal units or spores; these are set free, are mobile for a
while, eventually come to rest, and develop to the normal size. A hint,
however, of the beginning of a difference is seen when the cell occasionally
divides into a larger number of smaller spores. These, however, show no
difference in history. They settle down, and develop just like their more
THEORY OF SEX— ITS NATURE AND ORIGIN. I37
richly dowered neighbours. We find here the (Kcurrence of units of smaller
size, that is to say, less predominantly anabolic, but still these are able to
develop Independently.
(2.) In a higher alga, Ulothrix — one of the series known as Confervse —
both large and small reproductive cells are developed. The large ones
develop always of themselves, and so may the smaller forms. But the
smaller forms may also unite in pairs, and then start a new plant from the
double capital thus attained. When one of the smaller cells develops by
itself, the result, in some cases at least, is a weakly plant. They have what
Professor Vines calls an ** imperfect sexuality," for while they are in part
dependent upon union with other cells, they are not wholly so. They are
anabolic enough, we may say, sometimes to develop independently, but
often they are individually too katabolic for anything but weak independent
development. In uniting, however, in mutual nutrition, they are strong.
The student will already see the relative femaleness of the large units, the
maleness of their smaller neighlx)urs.
(3.) A third stage is reached in another alga, Ectocarpus^ which is
peculiarly instructive. 1 his may separate ofT large cells which develop by
themselves like parthenogenetic ova. From other parts of the plant
smaller units are liberated, which generally, though not yet invariably,
unite with one another before developing. But between these smaller
units a most important physiological difference has been observed by
Berthold. Some soon come to rest and settle dow:!, and with these their
more energetic neighbours by-and-by u:.Ite. We liave here a very distinct
beginning of the distinction between male and female elements. The
comparatively sluggish, more nutritive, preponderatingly anabolic cells,
which soon settle down — are female ; the more mobile, finally more
e.xhausted and emphatically katabolic cells — are male. As Vines says,
*' the one is passive, the other active; the former is to be regarded as the
female, and the latter as the male reproductive cell."
(4.) Further, in another alga, CtitUria^ the differentiation may be
traced. Two kinds of units result, which must unite with one another if
development is to take place, but these units arise from perfectly distinct
sources in the parent plant. The larger less mobile cells, which soon
come to rest, are fertilised by the smaller more active units. The more
anabolic or female cells are fertilised by the more katabolic or male cells,
which have now gone too far for the possibility of independent develop-
ment.
(5.} To complete the series, we may simply mention such a case as
that to which we shall presently return, — those forms of Fb/vax, where an
entire colony of cells produces either female or male elements, thus repre-
senting the beginning of an entirely unisexual many-celled organism.
While the above cases also involve the problem of the
origin of fertilisation, which is left over for the present, they
confirm our conclusion that relative predominance of kata-
bolism or anabolism is the characteristic of male or female
respectively.
§ 5. Nature of Sex as seen in Origin among Animals. —
Among the Protozoa also, we can trace the beginnings of the
138 THE EVOLUTION OF SEX.
same "dimorphism" between male and female. A union
between similar cells is of course frequent, but that is not at
present to the point. What we refer to are the numerous
cases, especially among flagellate and Vorticella-lilce infusorians,
where the two individuals which unite are quite unlike one
another both in form and history. " There can be no doubt,"
Hatchett Jackson remarks, "that the process is essentially a
sexual one; when the individuals are invariably different, there
is no reason why the terms male and female should not be
applied to them." In some cases we find as before that a
small active katabolic unit combines with a larger, more
passive, and anabolic individual.
In the bell -animalcule {Vorticeila\ so common on the
water-plants of our ponds, a minute free-swimming unit, formed
as one of the results of repeated division, unites with a stalked
individual of the normal size. In the related Episiylis^
Engelmann has described how an individual divides first of
all into two cells. One of these remains as such (like an
ovum), the other repeatedly divides (like a mother sperm-cell)
into numerous minute units. One of these subsequently
unites with the undivided cell, and Engelmann does not
hesitate to call the different elements male and female. In
some radiolarians {e.g.^ Coilozoum) dimorphic spores — large
and small — have been described, although their history has
not yet been fully traced.
As another illustration, it will be instructive to select the
case of Volvox. In this colonial organism, which is best
regarded as a multicellular protist, the component cells are at
first all alike. They are united by protoplasmic bridges, and
simply form a vegetative colony. In favourable environmental
conditions this state of affairs may persist, or be interrupted
only by parthenogenetic multiplication. When nutrition is
checked, however, sexual reproduction makes its appearance,
and that in a manner which illustrates most instructively the
differentiation of the two sets of elements. Some of the cells
are seen differentiating at the expense of others, accumulating
capital from their neighbours; and if their area of exploitation
be sufficiently large, emphatically anabolic cells or ova result ;
while if their area is reduced by the presence of numerous
competitors struggling to become ova, the result is the forma-
tion of smaller, less anabolic cells, which become ultimately
maicy segment into antherozoids, meantime losing their vegeta-
THEORV OF SEX— ITS NATURE AND ORIGIN. 139
tive greenness and becoming yellow. I ii some species, distinct
colonies may, in the same way, become predominantly anabolic
or katabolic, and be distinguished as completely female or
male colonies. Thus, again, we reach the conclusion, of a
predominant anabolism effecting the differentiation of female
elements, and of katabolism as characteristic of the male,
§ 6. Conclusion. — In conclusion, in defiance of Dr Minot's
dictum, that "such speculation passes far beyond the present
possibilities of science," we believe that the consideration {a)
the specal reproduclLve cells (a, J), both mate' and
remalc-Aftu Cohn.
of the characteristics of the sex-elements, alike in history, as
Minot himself emphasises, and in their finished form, {h) of
the incipient sex dimorphism seen among the simplest plants
and animals, (c) of phenomena, both normal and pathological,
in the sexual tissues and organs, {■/) of the established facts
in regard to the determination of sex (chap. 4), and («) of the
structural and functional, primary and secondary characteristics
of the sexes (chap. 3 and passim) — all lead to the conclusion,
that the female is the outcome and expression of relatively
140 THE EVOLUTION OF SEX.
preponderant anabolism, and the male of relatively pre-
dominant katabolism. Corroborations will gradually appear
in the succeeding sections, as we discuss fertilisation, partheno-
genesis, or special facts like menstruation and lactation.
The late Mr G. J. Romanes criticised our thesis as a mere
re-statement of the facts of sexual dimorphism without any
explanation of them. But no other course is open to such
inquiries but that of re stating. Our whole point is that of
re-stating the idea of dimorphism in protoplasmic terms, at a
deeper level of analysis than heretofore, in kinetic terms instead
of static ones, and in such a way that the origin and evolution
of sex are seen to be not phenomena per j^, . specialised, as
naturalists have commonly thought, from the rest of the
organism, but congruent with the origin and evolution of
other organic differences. From another point of view, we
may say that we are seeking to re-state the phenomena of sex
by a deeper recognition of the unity of the organism, and of
organisms.
Yi
THEORY OF SEX — ITS NATURE AND ORIGIN. I4I
SUMMARV,
1. Suggested theories of the nature of male and female; their number
and vagueness. Three recent developments — («) Rolph's penetrating sug-
gestion of more nutritive females, less nutritive males; {d) Minot*s theory
of the differentiation of both kinds of sex cells from a primitive hermaphro-
ditism ; {c) the conclusion of Brooks, that the males are more variable, and
alone transmit new variations.
2. Nature of sex seen in its essence in the sex-cells. The fundamental
protoplasmic antithesis illustrated in the Protozoa, in the cells of higher
animals, in life-histories. The conception of a cell-cycle. The physio-
logical import of this, — the protoplasmic possibilities, preponderant ana-
bolism, predominant katabolism, and a relative equilibrium. The analxjiic
character of the ova. The katalx)lic character of the sperms.
3. The problem of the origin of sex, so little tackled, because of (a) the
logical sufficiency of the selection theory and pre-occupation with inquiries
as to the utilitarian justification of the facts, {b) the numl)er of separate
problems involved, {c) the isolation of sex and reproduction from the
general life of the organism and species.
4. A series from simple plants, showing the gradual appearance of
dimorphic sex-cells, with the physiological interpretation thereof. The
dimorphism is the result of relatively preponderant katabolism and ana-
bolism, and this is the origin of male and female.
5. Illustrations of incipient dimorphism or sex among the Protozoa.
Special reference to the case of Volvox.
6. General conclusion, — (a) from the sex-cells, [b) from incipient sex, (c)
from organs and tissues, {d) from the determination of sex, {e) from the
characters of the sexts, — that male and female are the results and expres-
sions of relatively predominant katabolism and auabolism respectively.
LITERATURE.
Balbiani.— Lefons sur la g^n^ration des vertibr^s. Paris, 1879.
Brooks, W. K.— The Law of Heredity. Baltimore, 1883.
Dangeard, p. a. — Theorie de la Scxualite. Le Botaniste, Serie VI.,
1898.
ElGBNMANN, C. IL — Sex- differentiation in the viviparous Teleost Cyma-
togaster. Arch. Entwickelungsmechamk., IV., 1896, pp. 125-179,
6 pis., I fig.
Grddes, p. — Op. ciL^ especially "Theory of Growth, Reproduction, Sex,
and Heredity," Proc. Roy. Soc. Edin., 1886; and Article "Sex,"
Encyc. Brit., also " Resiaiement of Ceil Theory," Proc. Roy. Soc
Edin., 1883 84.
Hartog, M. — Some Problems of Reproduction. Quart. Journ. Micr,
Sci., XXXIIL, 1891, pp. 1-70.
Haycraft, J. B.— The Role of Sex. Natural Science, VIL, 1890.
Klbbs, G. — Ueber einige Problems der Physiologic der Fortpflanzung.
Jena, 1895, 26 pp. Die Bedingungen der Fortpflanzung bei einigen
Algen und Pilzen. Jena, 1896.
142 THE EVOLUTION OF SEX.
Lgndl, a. — Hypothese Uber die Entstehung von Soma- und Propagation-
zellen. 8vo. Berlin, 1889, 78 pp., 16 figs.
LiGNiBR, O. — Sur Torigine de la G^n^ralion et celle de la Sexuality.
Miscellanies Biologiques D^di^es au Prof. A. Giard. Paris, 1889,
pp. 396-401.
Maupas, £. — La Rajeunissement Karyogamique chez le Cili^s. Arch.
Zool. Exp^r., VII.. 1889.
MiNOT, C. S. — Theorie der Genoblasten. Biol. Centralbl., IT., p. 365.
Ueber Vererbung und VerjUngung. Biol. Centralbl., XV., 1895,
pp. 571-87.
NussBAUM, M. — Ziir Differenzirung des Geschlechts im Thierrcich. Arch.
Mikr. Anat., XVIII., 1880.
ROLPH, W. H. — Biologische Probleme. Leipzig, 1884.
Romanes, G. J.— Darwin and after Darwin. 1895-98.
Rydrr, J. A. — The Origin of Sex through Cumulative Integration, and
the Relation of Sexuality to the Genesis of Species. Amer. Philo.s.
Soc, XXVIII., 1890, pp. 109-159.
Sachs, J. — Text-book of Botany, edit, by Vines, second edition, 1882;
and Physiolc^ of Plants, translated by Marshall Ward, 1887.
Vines, S. H.— Physiology of Plants, 1886; article "Reproduction —
Vegetable," Encyc. Brit,
Weismann, a.— C?/. n/.
BOOK III.
M t M
PROCESSES OF REPRODUCTION.
f
.1
CHAPTER XL
Sexual Reproduction.
§ I. Different Modes of Reproduction, — It is well known that
a starfish deprived of an arm can replace this by a fresh growth ;
that crabs can renew the great claws which they have lost in
fighting; and that, even as high up as the lizards, the loss of
part of a leg or a tail can be made good. In a great variety
of cases, but decreasingly as we ascend from lower to higher
organisms, there is reparation of external injuries. Generally
speaking, the facts bear out Lessona's generalisation that re-
generative processes are adaptive, occurring in those animals
and in those parts of animals in which mutilation tends in the
natural course of life to be frequent. Now this "regenera-
tion,^' as it is called, is in a certain degree a process of repro-
duction. By continuous growth the cells of a persistent stump
are able to reproduce the entire member. We know too that
a sponge, a hydra, or a sea-anemone may be cut into pieces,
with the result that each fragment grows into a new organism.
The same is done with many plants; and though these multi-
plications are artificial, they illustrate what Spencer and Haeckel
said long ago, that reproduction is but more or less discon-
tinuous growth. So again, we pass onwards insensibly from
cases of continuous budding, as in sponge or rose-bush, to
discontinuous budding in hydra, zoophyte, and tiger-lily, where
the offspring, vegetatively produced, are sooner or later set free.
Similarly in the Protozoa, an almost mechanical breakage begins
the series. This becomes more definite, in the production of
several buds at once, or of only one. Budding leads on to
ordinary division, both multiple and binary: while finally, in
\ colonial forms, the liberation of special reproductive units may
be observed
I § 2. Facts involved in Sexual Reproduction. — It is neces-
I sary, at the outset, to be quite clear as to the occurrence of
several distinct facts in any ordinary case of sexual reproduction
! lO
.146 THE EVOLUTION OF SEX.
among many-celled organisms, (i.) There is, first of all, the
fact that special reproductive cells are present in more or less
marked contrast to the ordinary cells making up the body.
To this antithesis we have already given due prominence. (2.)
Then there is the further fact, that these special reproductive
cells are dimorphic; that they, and the organisms which pro-
duce them, are distinguishable as male and female. This has
been the main theme of the two preceding books. (3,) Lastly,
we have to recognise that these dimorphic sex-cells are mutually
dependent, — that if the egg-cell is to develop into an organism,
it must first be fertilised by a male element. On the facts
of fertilisation, therefore, as observed in plants and animals,
attention must now be concentrated.
§ 3. Fertilisation in Plants, — "The Newly Discovered
Secret of Nature in the Structure and Fertilisation of Flowers,"
so ran the title of a work published by Conrad Sprengel in
1793, embodying his pioneer investigations on a now familiar
field. Though not indeed the first to point out the importance
of insects in relation to fertilisation, — ^for that honour appears
to belong to Kolreuter (1761), — Sprengel laid sure foundations,
now somewhat hidden by the superstructure which Darwin and
others have built. To SprengeUs eyes, the many ways in which
the nectar is protected from rain seemed full of ** intention."
He recognised in the markings of the petals illumined finger-
posts to lead insects to the hidden hoards; and he further
demonstrated that in some bisexual flowers it was physically
impossible for the pollen from the stamens to pass to the tips
of the carpels. His general conclusion, freely stated, was, that
** since a large number of flowers have the sexes separate, and
probably at least as many hermaphrodites have the stamens
and carpels ripening at diflerent times, nature appears to have
designed that no flower shall be fertilised by its own pollen."
A few years later (1799), Andrew Knight maintained that no
hermaphrodite flower fertilises itself for a perpetuity of genera-
tions.
Sprengel's secret of nature had, however, to be set forth
afresh by Darwin, who, in his "Fertilisation of Orchids"
(1862), and "Efl*ects of Cross- and Self-Fertilisation" (1876),
has not only shown, with great w^ealth of illustration, the mani-
fold devices for ensuring that insects unconsciously carry the
fertilising pollen from one flower to another, but has also
emphasised the advantage of cross-fertilisation for the health
SEXUAL REPRODUCriOTT. I47
of the species "Nature tells us," he says, "in the most
emphatic manner that she abhors perpetual self-fertilisation."
Hildebrand, Hermann Miiller, Delpino, and others, have, with
consummate patience of observation, further traced out the
secrets of nature in this relation; and the student may be
referred to D'Arcy Thompson's valuable edition of Miiller's
" FerliUsation of Flowers," Sir John Lubbock's " Flowers in
Relation to Insects," the classic works of Darwin, and P.
Knuth's "Handbuch der Blu ten biologic," 2 vols., Leipzig,
1892. Reference must, however, also be made to Meehan's
protest (see pp. 80, 81), that self-fertilisation is neither so rare
nor so "abhorrent " as is generally believed.
In a great number of cases, cross-fertihsation by means of
insects does occur; in many it must occur. In another by no
Stt, vlsiiing WhiK D<adntii1= (B), and ll^oom (A).
means small set of flowering plants, — usually with inconspicuous
blossoms,— the fertilising gold dust is borne by the wind, and
falls, like the golden shower on Danae, upon adjacent flowers.
In many hermaphrodite flowers, again, self-fertilisation does
certainly lake place; in some this is necessarily so. Indubi-
table self-fertilisation occurs in the small degenerate unopening
(cleistogamous) flowers of some plants, such as species of
balsam, deadnettle, pansy, &c. These occur along with
ordinary flowers, and, curiously enough, are sometimes more
fertile than they.
In most of the lower plants, the male elements are minute,
and actively mobile. They find their way through the water,
or along capillary spaces between the leaves, to the passive
female cells. In some cases there is a curvature of the male
148 THE EVOLUTION OP SEX.
organ towards an adjacent female organ, apparently in obedi-
ence to chemical or physical attraction. Even here close
fertilisation seems exceptional, and is often impossible.
So fat, however, only the external aspect of the process.
So long ago as 1694, Camerarius showed that if the male
flowers of hemp, maize, and other plants were removed, the
female flowers bore no seeds, or at least no fertile ones. In
1704, E. F. GeoiTroy castrated certain plants by removing the
stamens, and noted that they remained barren. "Mirandum
sane," he wrote, "quam similem servet natura cunctis in
viventibus generandis harmoniam." Reasonable as this now
appears to us, the fundamental fact was not only slowly recog-
nised, but on into the present century there were found
A, Enlaistd Hclion of ripe Aolbtr (f\ liberalinj polkn (a). B, Dugnnnnuilii!
onuy "ilh Hd (rf); ihe sumtns {i) will, pt^tn! d Tha''pollfn'luho
(a) growing down tu the ovule {d) u>d female cell (t). The pollin-grain a
here represeDled u dulinclly Iwo-Cciled.
naturalists who strongly opposed it, and denied the sexuality of
plants altogether. In 1830, however, Amici made a great step.
He traced the pollen-grain from its lighting on the carpel tip
down into the recesses of the ovule. Schleiden, whose name is
so closely associated with the founding of the "cell theory,"
Boon confirmed Amici's observation, but in doing so went
unfortunately much too far. Not only did the pollen-grain
send its tube into the ovule, but there, according to Schleiden,
it gave origin to the future embryo. This opinion, which, as
Heyer observes, made the male element really female, was
obviously parallel to that of the zoologists who found in the
"soerm-animalcule" the miniature embryo. The view of
SEXUAL REPRODUCTION. I49
Camerarius and Amici of course prevailed ; and we now know
not only the fact that the pollen-grain is a male element which
unites in fertilisation with a female cell, but, thanks especially
to Strasburger, much about the intimate nature of the process.
In the last century, Millington emphasised the difference
between male and female flowers, and we can trace the
influence of this discovery in Erasmus Darwin's " Loves of the
Plants."
In the last few decennia, it has been shown, for many of the
lower plants, that fertilisation essentially involves the union of
the nuclei of male and female cells. By analogy the same was
believed to be true of higher plants, but direct demonstration
has only recently been forthcoming. Strasburger has followed
the whole history of the pollen-grain, from the anther of the
stamen to the embryo-sac of the carpel ; and though some details
still remain obscure, his researches have undoubtedly succeeded
in elucidating the essential facts in the process. He shows
how the pollen-grain divides into a vegetative and a generative
cell, of which only the latter is directly important in fertilisation.
The generative cell, which consists like the sperm mostly
of nucleus with very little directly associated cell-substance,
itself divides to form two (or even more) generative nuclei
One of these passes from the pollen-tube to enter into close
union with the nucleus of the female cell, with which it fuses
to form the double nucleus ruling the forthcoming develop-
ment. Exceptionally the other generative nucleus may also
unite with the nucleus of the egg-cell, but this is almost as rare
as "polyspermy" among animals. According to Strasburger,
the cell-substance of the pollen-grain or pollen-tube which sur-
rounds the nucleus has no direct influence in the essential act
Fertilisation is a union of two nuclei, "the cell-substance of the
pollen-tube is only the vehicle." He confirms the observations
of PfeflTer, as to the reality of an osmotic attraction between at
least the surroundings of the two essential elements, in accord-
ance with which the pollen-tube bearing the generative nucleus
is marvellously guided to its destination. The differentiation
of the generative nucleus, in contrast to the more vegetative^
and the true nuclear union which forms the climax of fertilisa-
tion, are two very important facts, showing the unity of the
process not only in higher and lower plants but in all
organisms.
Although there are peculiarities distinguishing the processes
ISO THE EVOLUTION OF SEX.
of maturation and fertilisation in plants from those observed in
animals, it is impossible to deny the essential parallelism. A
discussion of this would lead us into technical details, which
cannot be profitably described without abundant figures, but
we may refer, for instance, to a comparative survey by Professor
V. Haecker ("BioL Centralblatt," xvii., 1897, pp. 689-705,
721-745, 40 figs.). One of the most striking botanical dis-
coveries of recent years is the fact demonstrated by Hirase and
Ikeno, that in the Ginkho {Saiisburia adianiifolid)^ and in
Cycas revoluta^ — gymnosperm flowering plants belonging to
the family of Cycads, — the male nuclei issue from the pollen-
tube as motile spermatozoa or antherozoids. Webber has also
announced a similar discovery in the case of Zamia integrifoUa,
§ 4. Fertilisation in Animals. — That the sperms are essential
to fertilisation was a conclusion by no means recognised when
those elements were first seen. Gradually, however, the fact
was demonstrated, both by experiment and observation. Jacobi
(1764) artificially fertilised the ova of salmon and trout with
the milt of these forms, and somewhat later the Abb^ Spallan-
zani extended these experiments to frogs and even higher
animals. Even he, however, believed that the seminal fluid
was the essential factor, not the contained spermatozoa.
Through the experiments of Provost and Dumas (1824),
Leuckart (1849), and others, attention was directed to the
real import of the sperms, which Kolliker referred to their
cellular origin in the testes. The presence of the sperm within
the ovum was observed in the rabbit ovum by Martin Barry in
1843; by Warneck, in 1850, for the water-snail, a fact con-
flrmed about ten years afterwards by Bischoff and Meissner;
in the frog ovum by Newport (1854); and in successive years
it was gradually recognised in a great variety of animals.
The adaptations which secure that the sperms shall reach
the ova are very varied. Sometimes it seems almost a matter
of chance, for the sperms from adjacent males may simply be
washed into the female, as in sponges and bivalves, with the
nutritive water-currents. In other cases, especially well seen
in most fishes, the female deposits her unfertilised ova in the
watery the male follows and covers them with spermatozoa.
Many may have watched from a bridge the female salmon
ploughing along the gravelly river bed depositing her ova,
careful to secure a suitable ground, yet not disturbing the
already laid eggs of her neighbours. Meanwhile she is attended
SEXUAL kEPRODUCTIOi»J. I5I
by her (frequently much smaller) mate, who deposits milt upon
the ova. In the frog, again, the eggs are fertilised externally
by the male just as they leave the body of his embraced mate.
Or it may be that the sperms are lodged in special packets,
which are taken up by the female in most of the newts, sur-
rounded with one of the male arms in many cuttle-fishes, or
passed from one of the spider's palps to the female aperture.
In the majority of animals, e.g,^ insects and higher vertebrates,
copulation occurs, and the sperms pass from the male directly
to the female. Even then the history is very varied. They
may pass into special receptacles, as in insects, to be used
as occasion demands ; or, in higher animals, they may with
persistent locomotor energy work their way up the female
ducts. There they may soon meet and fertilise ova which
have been liberated from the ovary; or may persist, as we
noticed, for a prolonged period ; or may eventually perish.
When the sperms have come, in any of these varied ways,
into close proximity to the ovum, there is every reason to
believe that a strong osmotic attraction is set up between the
two kinds of elements. We have often suspected that the
approach of the conjugating cells of two Spirogyra filaments
might be directed along, the line of an osmotic current; and
although we must confess that perhaps somewhat rough
evaporations, performed a few summers ago, gave no positive
confirmation to the idea that glucose or the like might be
present in appreciable quantity in the water, a recent observer,
we are glad to see, claims to have been more fortunate.
The spermatozoa, which seem so well to deserve Rolph's
epithet of "starved," appear to be powerfully drawn to the
well-nourished ovum, and the latter frequently rises to meet
the sperm in a small ** attractive cone." Often, however,
there is an obstacle in the way of entrance in the form of the
egg-shell, which may be penetrable only at one spot, well
called the micropyle. Dewitz has made the interesting observa-
tion that round the egg-shells of the cockroach ova, the sperms
move in regular circles of ever-varying orbit; and points out
that thus, sooner or later, a sperm must hit upon the entrance.
He showed that this was a characteristic motion of these
elements on smooth spheres, for round empty egg-shells or on
similar vesicles they moved in an equally orderly and systematic
fashion.
The persistence with which the spermatozoa often force their
ISi THE EVOLUTION OF SE^i.
way to the ova makes it impo sible to doubt the reality of a
strong chemotactic attraction. One illustration may suffice.
According to Dr Sadone's account of the impregnation in
the Rotifer Hydatina senta ("Zool. Anzeiger," xx., 1897,
pp. 513-17), the spermatozoa of the male, which are injected
into the body-cavity of the female, reach the totally enclosed
jcggs by boring through the thin membrane at a point where
the mature ova are situated — a process not known in any
other animals. The oval head of a spermatozoon was seen to
attach itself to the membrane of the ovary, the tail continued
to make lashing movements, the head was gradually forced
through the membrane, and the tail followed, the whole process
taking about ten minutes.
It was till recently believed that more than one sperm
might at least enter the ovum, but researches such as those
of Hertwig and Fol have shown that when one sperm has
found admittance, the way is usually barred against others.
The micropyle may be blocked, or the surrounding membrane
may be altered, or in other ways the ovum may exhibit what
Whitman calls "self-regulating receptivity," so as to be no
longer penetrable. We are safe in concluding, — that the ovum
is usually receptive only to one sperm; that in most cases the
entrance of more than one sperm is impossible; and that where
"polyspermy" does occur, pathological development is at
least often the result.
It may be well to note that there are, as in plants, various
steps in the process which is often roughly summed up in the
one word — fertilisation.
(1) There is the process by which the spermatozoa
are brought into general proximity to the ova. In higher
animals this is best termed insemination, and is accomplished
by copulation.
(2) There is the approach of the spermatozoon to the ovum,
but of this little is known.
(3) There is fertilisation in the strict sense, — the intimate
and orderly union of two sex -nuclei.
What takes place before fertilisation is, as we have just seen, very
varied indeed among animals; what takes place after fertilisation is of
course cell division, but that, though referable to certain great types, must
necessarily be very diverse ; what takes place in the act of fertilisation,
however, is always essentially the same. The head of the spermatozoon
becomes the male nucleus (or -|)ro-nucleus) of the fertilised ovum, entering
into close association with the female nucleus. The latter, as we have
Sexual kEt»RODUCTioK. 153
already noted, has had its own history; it is no longer the original ger-
minal vesicle, nor usually like it in appearance, it is the germinal vesicle
minus the quantity of nuclear substance given oH* in forming two polar
globules. This female nucleus (or pro-nucleus, as it is generally called)
comes into close association with the sperm or male- nucleus ; nor does it
remain quite passive in the process, though the greater activity in bringing'
about the close association is certainly still exhibited by the male. Whit-
man has recently emphasised the reality of an attractive influence between
the pro-nuclei. ?'usion of the pro-nuclei was observed so long ago as
1850 by Warneck in the pond-snail (Lymnaus), The result, however,
appears to have been overlooked, till the same fact was reobserved in
threadworm ova by BUtschli in 1874. Since that date the fact has been
continuously studied. Some observers still doubt whether what can be
accurately called fusion of nuclei ever occurs; and if fusion means inextric-
able confounding and mixing up of the male and female nuclear elements,
it is almost certain that such does not in any case happ>en. There is no
doubt, however, that the two nuclei become very closely associated, and
according to most observers a double unity is formed, in which the com-
ponent nuclear elements from the two origins so diverse are united in
perfectly orderly fashion. So exact, in fact, is this duality, that when the
first division of the egg takes place, each of the two daughter-cells has
in its nucleus half of the male and half of the female elements, and so on
perhaps in after-stages.
The object upon which the intimate phenomena of fertilisation have
been most studied is the ovum of the threadworm {Ascaris megalocepkala)
which infests the horse. Since 1883 numerous important memoirs have
dealt with this subject, and with the same material. The general results
stand out clearly, though there remain not a few minor discrepancies. To
one of these, now explained, we may briefly refer. According to Van
Beneden, the normal ovum of the threadworm contained in its nucleus one
chromatin element, and was fertilised by a sperm also with one chromatin
element. Carnoy, however, described the normal ovum as containing two
chromatin elements, and as fertilised by a sperm also with two. In view
of the perfection with which both these investigators had unravelled the
structure and behaviour of the nuclei, the discrepancy seemed serious
enough. But Boveri has shown that both are right ; Van Beneden's type
occurs; Carnoy's type also occurs. Nay more, an ovum with one chromatin
element seems to be always fertilised by a sperm with only one, while an
ovum with two chromatin elements is fertilised by a sperm likewise with two.
A few of the details may be summarised from the masterly researches of
Boveri. The extrusion of the two polar cells from the ovum is in reality a
double process of cell-division. The quantity of the nuclear substance in the
germinal vesicle is thereby reduced, and the number of nuclear elements is
also reduced to half the normal. Only one sperm penetrates the ovum,
unless the latter be unhealthy ; and with the entrance of the sperm the ovum
undergoes a simultaneous change, which excludes other male elements.
Only the head or nuclear portion of the sperm is of real importance in the
essential act of fertilisation ; the nutritive tail or cap simply dissolves away.
After the sperm-nucleus has penetrated to the centre of the ovum, and
after the extrusion of the polar bodies is quite completed, we have to deal
with two nuclei, not only closely approximate in structure, but alike in
further history.
154 THE EVOLUTION OP SEX.
. 'ype. I " "
elements, in the form of bent rods; and Iwfore union takes place, the-ie
undergo a marked modiRi^lion, the same in both cases. Round the chro-
matin tods vacuoles are formed, limiting ihem from the surrounding
protoplasm ; into these the rods send out anastomosing processes, after
the fashion of little rhizopoda ; gradually the rods thus resolve them-
selves into a network, in the meshes of which minute "nucleoli" are
also demonstrable.
I I
Daaiain of Iht Piocea of Finil^lion, aHer Bovtri.— a, feouie pio-nudeus; *, pnbT
The (wo nuclei thus modified then unite, but that again so precisely, as
Van Beneden es|)ecially has shown, that each forms half of that spmdle
figure which almost all nuclei take when about to divide. This double
spmdle figure is the "segmentation nucleus," which will presently divide
info the two first daughiernuclei of the ovum (see figs. VI.-X.).
It is not possible here to discuss certain intricate changes which late
place meanwhile, not in the nuclei, but in the eel I -substance of the ovum.
Both Van Beneden and Boveri have recently agreed on
SEXUAL REPRODUCTION.
156 THE EVOLUTION OF SEX.
"central corpuscles" (centrosomata) in the protoplasm. These serve as
*' points of insertion" for protoplasmic threads, which exert a ** muscular
action" upon the nuclear elements in the forthcoming division. Boveri
has traced with great care the history of a special kind of protoplasm (what
he calls the archoplasm), which has its centre in either " central corpuscle"
(^), and sends out fibrils (/), which moor themselves to the nuclear ele-
ments. The movements of the latter during the forthcoming first division
of the ovum are directly referable to the antagonistic action of these fibrils,
and thus we have hints of an intracellular muscularity.
In the spindle the nuclear elements, still distinguishable in their orderly
behaviour as male and female, eventually form what is known as the
*' equatorial plate " (VI.), lying across the centre of the spindle. This is a
well-marked stage, and one characterised by apparent equilibrium. ** It is
the resting-stage par excellence in the life of the cell. Movement is at an
end, a state of stability has set in, and this would continue ad infinitum,
did not a factor, which hitherto has played no part, assert itself and bring
about fresh movement. This new movement is the longitudinal division of
the chromatin elements, an independent expression of life — indeed, a re-
productive act—on the part of the nuclear elements."
Of each longitudinally split chromosome one half moves or is moved
towards the one centrosonie, and the other half towards the other centro-
some. After this apparently equal partition a nuclear reconstruction is
gradually effected, and the ovum reaches the 2-cell stage.
One marvellous fact, showing the closeness of union in
fertilisation, may be briefly re -emphasised. In the double
nucleus formed from the union of male and female nuclei, Van
Beneden, Carnoy, and others, have shown that both constituents
have an equal share. The one half is paternal, the other
maternal, and this is true not only for Ascaris (Van Beneden)
and other threadworms (Carnoy), but for representatives of
other worm-types, coelenterates, echinoderms, molluscs, and
tunicates. In the division which forms the first two daughter-
cells (IX., X.), half of each set of constituents goes to either
cell, and the dualism is kept up. Furthermore, it is probable
that of the chromatin loops observed in the division figure of a
daughter-cell, half are derived from the male parent, and half
from the female. The importance of this fact, in relation to
the influence of both parents upon the offspring, is very
obvious.
One of the clearest of modern exponents. Professor E. B.
Wilson, who has himself made important contributions to the
subject, sums up the present-day view of the matter in the
following sentences: — "From the mother comes in the main
the cytoplasm of the embryonic body, which is the principal
substratum of growth and differentiation. From both parents
comes the hereditary basis or chromatin by which these pro-
SEXUAL REPRODUCTION. 157
cesses are controlled, and from which they receive the specific
stamp of the race. From the father comes the centrosome to
organise the machinery of mitotic division by which the egg
splits up into the elements of the tissues and by which each of
these elements receives its quota of the common heritage of
chromatin. Huxley hit the mark two score years ago when he
compared the organism to a web, of which the warp is derived
from the female and the woof from the male. What has since
been gained is the knowledge that this web is to be sought in
the chromatic substance of the nuclei, and that the centrosome
is the weaver at the loom.*' (See " The Cell in Development
and Inheritance," 1896, p. 171.)
The above short sketch will show how intricate, and yet at
the same time how orderly, are the intimate processes of fer-
tilisation. Variations do indeed occur, both in pathological
and in apparently normal cases; but a general constancy is
now both clear and certain, not only for many different animals,
but also to a certain extent, as Strasburger and others have
shown, for plants.
§ 5. Fertilisaiion in Protozoa. — In the nascent sexual union observed
in many Protozoa, considerable diversity obtains. The individuals which
unite may be to all appearance similar (to which cases the term conjugation
is generally applied), or they may be materially dimorphic, as in Vorticella,
The union may be permanent, when the two units fuse into one ; or it may
only be temporary, during which an interchange of elements takes place.
The union may be complete, as in the conjugation of two Gregarines, or
partial, as in the slipper-animalcule, and between these may be placed the
state of affairs observed in various species of bell-animalcule ( Voriicella)^
where the nuclei and the bulk of the smaller conjugate pass into the larger,
leaving, however, shrivelled remains which are cast off. This, as described
by Hs. Wallengren ("Biol. Centralblatt," xix., 1899, pp. 153-161,
3 figs. ), shows that the distinction between total and partial conjugation is
only one of degree. In many cases the nuclear elements are seen to play
an important part, disrupting and reconstructing during the process, while
a genuine flision of the two nuclei has also been observed in permanent
conjugation.
In r^ard to the interchange of elements, there is considerable diver-
gence of observation. Joseph has noted what appears to be an interchange
of protoplasm ; Schneider has observed the exchange of nuclear elements ;
while Gruber and Maupas, and Joseph as well, have, in their studies on the
union of ciliated infusorians, laid emphasis on an accessory nuclear body,
generally known as the " micro-nucleus." This body lies by the side of the
larger nucleus, and while the latter simply disrupts and dissolves away, or
is extruded without playing any important part, the smaller micro-nucleus
divides in a regular way, and with the results there is micro-nuclear inter-
change between the two individuals.
According to Maupas, who has investigated the subject in most detail,
158 THE EVOLUTION OF SEX.
♦
the para- or micro-nucleus is a *' hermaphrodite" sexual element, of sole
importance in conjugation. The stages in the process of fertilisation are as
follows : —
(i.) The micro-nucleus increases in size.
(2.) Division occurs until there are eight micro-nuclei.
(3.) Of these eight, seven disappear.
(4.) The remaining one divides again, differentiating a male and female
pro-nucleus.
(5.) In the next stage, the male elements of the two individuals are
exchanged, and the new male nucleus fuses with the original
female portion.
(6, 7.) In two following stages, the nuclear dualism characteristic of the
ciliated infusorians is re-established. The old large nucleus
(macro-nucleus) has broken up and been eliminated meanwhile.
(8.) Finally, the individuals, separating from one another, reassume all
their original organisation before beginning again to divide in the
usual fashion.
The union of the male and female nuclear elements in ciliate infusorians
was admirably figured by Balbiani so long ago as 1858; and though he does
not seem rightly to have interpreted what he observed in this particular
case, he was right in his contention that sexual union and fertilisation really
occurred in the Protozoa. Balbiani's view has been for long scouted, and
yet, with renewed observation, naturalists have now come back to his con-
clusion. Maupas willingly allows that Balbiani figured beautifully what he
himself has since reobserved and interpreted.
The phenomena described by Maupas, as summarised above, have been
observed in towards a dozen ciliated infusorians, so that there is every
reason to believe in their general occurrence. In three species of the
slipper-animalcule (Paramacium), and in species of Stylonichia^ Leu-
cophrys^ EuploUs, Onychodromus^ Spirosiomum^ &c., the facts are as above
stated.
It is of interest to cite the facts in regard to the common bell-animalcule
{Voriicelia)^ because here the conjugating individuals are like ovum and
sperm in more ways than one. In some species — e.g.,^ V, rncnilata^xYiQ
adult divides equally, to form two small individuals, which conjugate with
those of normal size. In V, microstoma there is again division into two,
but the products are of unequal size; one is much smaller than the other.
In the nearly allied Carchesium polypinum^ the divisions are equal, but
they are repeated twice or thrice. The result in all cases is the production
of minute mdividuals, which eventually attach themselves to adults of the
normal size, first to the stalk, and then to the body. The accessory nuclear
bodies divide as usual ; the large individual ceases to feed, and hermetic-
ally closes its mouth, like an ovum when fertilised. The small individual
is gradually absorbed by the larger, as sperm by ovum; and in an intricate
but orderly fashion a mixed nucleus results from the fusion of the micro-
nuclear elements of the two. The adult then begins to feed, to divide,
and so on, as usual. Here then there is (a) incipient dimorphism, {jb)
absorption of smaller by larger, and (r) intimate nuclear union, — facts which
we have already emphasised in the fertilisation of multicellular animals.
§ 6. Origin of Fertilisation, — To understand the origin of
the union of sex-cells, attention must still be concentrated on
SEXUAL REPRODUCTION. 1 59
the Protozoa. That fertilisation really occurs at that low level
in a highly complex fashion, we have just seen. It is necessary,
however, to note the steps which lead up to what Maupas and
others have so patiently elucidated.
(a) In the primitive life-cycle exhibited by Protornyxa (see
fig. at p. 129), the units which burst forth from the cyst sink
down into tiny amoebae, and unite together in numbers to form
a composite spreading mass of protoplasm, technically known
as a Plasmodium, This is undoubtedly a very primitive union
of cells, yet it occurs at very diverse levels in the organic series.
It is more or less familiar in the " flowers of tan," one of the
lowly Myxomycetes, where a nucleated mass of protoplasm,
of composite origin, spreads over the bark in the tan-yard.
The plasmodial union also occurs as a definite stage in the life-
history of the primitive neighbours of Protomyxa^ the Monera
of Haeckel. Pour the liquid contents or body-cavity fluid of a
freshly-dredged and still actively living sea-urchin into a bowl;
the cells which float in it, like blood-corpuscles in the blood,
draw together in clotted masses. Watch the process under a
microscope, and the formation of a plasmodium is seen. The
dying cells fuse into composite masses, just like the units of
Protomyxa; and it is interesting to observe that, though they
are dying, the union provokes a brief but intense renewal of
amoeboid activity. To forestall our point, they as it were
fertilise each other in articulo mortis. In spite of the objection
of Michel and others, that such union, being pathological, is
not comparable to the multiple conjugation normal to the
myxomycete, we maintain the distinct analogy between the
Plasmodium formation in Myxomycetes and that exhibited by
the cells in the body-cavity fluid of many animals, and regard
this as so much additional evidence of the profound unity of
the normal and the pathological processes. Now it is from
this primitive union of cells, as illustrated in the lowest organ-
isms, that we start in explaining the origin of fertilisation.
Just as the very beginning of reproduction seems almost like
mechanical breakage, so the very beginning of fertilisation is
found in the almost mechanical flowing together of exhausted
cells.
{p) Between this and the process usually described as con-
jugation, there are some interesting links. Sometimes as many
as three or four spores of lowly Algae club together, as if to
gather sufficient momentum to make a combined start in life.
i6o
THE EVOLUTION OF SEX.
The young forms of the sun-animalcule (Actinosphariuni)
usually unite in twos, but Gabriel has observed in some cases
a multiple union. In another sun-animalcule {Actinophrys sol)
two to thirty individuals may unite loosely in what is often
called plastogam}', but close union (of nuclei) only occurs
t)etween two individuals. So in gregarines (common parasites
in invertebrates), while the usual union is certainly dual, Gruber
has again observed what may be termed multiple conjugation.
Union of three has also been observed as an exception in
^ %
hp^-^.
Dia^ammatic representation of the stages
in the origin of fertilisation, — (I.) plas-
modium; (II.) multiple conjugation;
(III.) ordinary conjugation; (I V.) con-
jugation of dimorphic cells ; (V.) fertilisa-
tion of ovum by spermatozoon.
several infusorians. The union of more than two may thus be
interpreted as intermediate between the formation of plasmodia
and the normal dual conjugation.
(^) Conjugation of two similar unicellular organisms occurs,
as we have seen, very generally in the Protozoa, and is also a
common fact in the life-history of simple Algae. It is open to
every one possessed of a microscope to observe what conjuga-
tion means in such a common fresh-water alga as Spirogyra,
Opposite cells of adjacent filaments are attracted to one another,
and the contents of the one cell pass bodily over into the other.
SEXUAL REPRODUCTION. l6l
In the great majority of cases where conjugation occurs, the
uniting cells are to all appearance similar, but it must be
remembered that it does not follow from this that they are
physiologically alike.
(//) Both among plants and animals, all naturalists are
agreed that it is impossible to draw any line between the con-
jugation of similar and the union of more or less dimorphic
elements. " This differentiation presents," Sachs says, " espe-
cially in Algae, a most complete series of gradations between
the conjugation of similar cells and the fertilisation of oospheres
by antherozoids, any boundary line between these two processes
being unnatural and artificial" The gradual appearance of
Diagrammatic representation of tlie contrast
between coniugation (horizontal line) and
fertilisation (vertical line).
dimorphism has been already noted in discussing the origin
of sex, and need not be re-emphasised.
(e) Lastly, in fertilisation among higher plants and animals,
the two elements which unite are highly differentiated, alike in
contrast to one another and in opposition to the general cells
of the body. A consideration of the phenomena in loose
protist colonies like Volvox or Ampullina^ which suggest the
bridge between unicellular and multicellular organisms, shows
how gradually this latter contrast also may have been brought
about.
To sum up, the steps in the development of the process of
fertilisation may be arranged in the following series : —
(a) The formation of plasmodia.
\b) Multiple conjugation.
II
1 62 THE EVOLUTION OF SEX.
(c) Conjugation of two similar cells.
(d) Union of incipiently dimorphic cells.
(e) Fertilisation by differentiated sex-elements.
One difficulty must in fairness be allowed in connection
with the hypothesis of deriving conjugation from plasmodial
union. Some years ago, Sachs was inclined to regard the
Plasmodium formation of Myxomycetes as a process of multiple
conjugation, but he afterwards withdrew this mainly on the
ground that the nuclei have not been shown to coalesce. Now
there seems no result of studies on fertilisation more certain
than that the union of nuclei is an essential fact, but in Plasmo-
dium formation, such intimate association of nuclei cannot be
asserted. The difficulty of making this a starting-point is thus
at first sight considerable.
Yet it must be observed, (i) that our knowledge of the
nuclei in those lowly forms is still very inadequate ; (2) that,
according to Gruber, the behaviour of the nucleus is sometimes
masked by the fact that, instead of existing as a discrete body
in the cell, it lies diffusely in the protoplasm ; but especially
(3) that it is quite consistent with the general evolutionary con-
ception to suppose that the primitive union was of very much
less definite character than that subsequently evolved.
Even in conjugation nuclear union is not always clear. It
is well known in the conjugation of Infusorians, but it has been
very rarely proved in other Protozoa. It has been observed,
however, in some cases, e,g,, by Wolters, in the common
MonocysU's of the earthworm, and by Schaudinn, in the sun-
animalcule, Actinophrys soL
Rhumbler has made an elaborate study of the possible
evolution of fertilisation-processes. He finds the first step
in cytotropism, in which chemotropic substances are secreted
between cells, and in this connection we must bear in
mind the experiments of Klebs, which show that the addi-
tion of various reagents to the culture-solution in which a
simple Alga or Mould is living will determine the occurrence
of sexual or asexual reproduction. The next step is plasto-
gamy, — two naked cells become apposed and fuse. At a
higher level, karyogamy is reached. (" Biol. Centralbl,"
xviii., 1898, pp. 21-26, 33-38, 69-86, 113-130, 14 figs.).
§7. Hybridisation in Animals, — Many of the compound names of
animals, such as leopard, point back to a once prevalent belief that animals
of very different kmds might unite sexually and have fertile offsprinfi^.
SEXUAL REPRODUCTION. 1 63
Only to a very limited extent is such a notion justified. Every one is aware
that by direct human control animals like horse and ass, dog and wolf,
lion and tiger, hare and rabbit, canary and finch, pheasant and hen,' goose
and swan, have been successfully crossed. In nature, however, we know
relatively litllc of the occurrence and results of any such hybridisation.
It seems to occur in some fishes; different species of toad are often seen
in sexual union; it is said to l)c not uncommon between various species of
birds and insects.
M. Andre Suchetct, after many years' study of hybridism in birds and
mammals, stated the following provisional conclusions ('* Journ. de I'Anat.
Physiol.," xxxiii., 1897, pp. 326-355):—
(1) Cases of hybridism in mammals number about 93, of which 82 are
{a) crosses of species of the same genus, and 1 1 are {d) doubtful cases of
crosses between members of different genera. There is no instance {c) of
crossing between members of distinct ramilies or widely-separated genera.
Among birds, 262 cases are recorded, 178 of the first category (a), 68 of the
second (^), and 16 (some doubtful) of the third {c).
(2) Of the 82 crosses between mammals of distinct species but of the
same genus, the great majority (62) resulted in sterile offspring. In about
1 2 cases, the offspring have proved fertile with one of the parent species (jr
wiih a third species. In 7 or 8 cases the offspring have proved fertile t'li/er
se, sometimes for three or four generations.
Among birds, in 178 crosses between members of distinct species but
of the same genus, only 22 resulted in fertile offspring, 8 inter se, the
others with the parent species, or with a third species, or with other
hybrids.
Of the 68 crosses l>etween species of difTerent genera but of the same
family, only one had off^])ring fertile with one of the parent species — the
male hybrid of Coiumba livia x Turtur t isorius was fertile with the female
of the latter species. The female hybrid resulting from the same cross
seemed sterile. In two other cases a hybrid of this category fertilised a
third species ; in another case it was fertilised by this third species.
(3) As to the causes of the sterility in the hybrid offspring, the repro-
ductive organs are sometimes atrophied, in other cases the ducts are
abnormal, but there remain many instances in regard to which we can only
shroud our ignorance with the word " constitutional."
In some cases hybridisation succeeds readily, in other cases it is very
difficult to bring it about Thus Mr II. M. Vernon found that in some
Echinoderms — Spharechinus and Strongylocaiirotus //V/Ir/wj^hybridisalion
occurred very readily and was highly successful. In some nearly related
frogs, on the other hand, it always fails. In certain cases the cause of the
difficulty is almost mechanical ; thus PHiiger showed that the spermatozoa
of Ranafuscay which have very pointed heads, thinner than those of related
forms, can fertilise the eggs of nearly all other species (A', arvalisy A*, escu-
Unta^ and Bufo communis)^ but the blunt, thick-headed spermatozoa of
/\. arvaiis and R, escuienta cannot fertilise the eggs of any other species.
On the other hand, Hertwig's experiments on sea-urchins point to the
conclusion that the state of the egg is very important in determining
whether the hybridising will succeed or not. Eggs in good condition
resist the entrance of spermatozoa to which the stale ova prove receptive.
It should also be noticed that fertilisation and segmentation may occur
without further development. This was Born*s experience in many cases
164 THE BV0LX7TI0N OF SEX.
with amphibians, and T. IL Morgan had the same result with the ova of
a starfish fertilised by the spermatozoa of a sea-urchin ; segmentation pro-
ceeded, a hybrid gastrula was formed, but no further progress was made.
There is no doubt that at least many species-hybrids tend to sterility,
but this is exhibited in varying degrees. The male mules are always
sterile, but some say that the females may be successfully impregnated by
horse or ass. In many cases hybrids are not fertile with one another, but
remain so with the parent form. In a few cases the reproductive functions
seem for a time at least to be exaggerated rather than diminished as the
result of crossing.
It seems also certain that while variety-hybrids are usually fertile, their
constitution is often unstable. They are often very variable, and apt to
die out, as has been repeatedly observed in the human species. The ill-
natured saying, ** God made the white man, God made the black man, the
devil made the mulatto," refers to the frequently inconvenient variability
of those variety-hybrids. It is impossible, however, to generalise this.
All that can be said is that some cross-fertilisations are very disadvan-
tageous, while others seem to be as markedly the reverse.
Brooks has laid considerable emphasis on the variability of hybrids in
connection with his theory of heredity. *' Hybrids and mongrels," he
says, ** are highly variable, as we should expect from the fact that many
of the cells of their bodies must be placed under unnatural conditions, and
must therefore have a tendency to throw off gemmules." " Hybrids, from
forms which have been long cultivated or domesticated, are more variable
than those from wild species or varieties, and the children of hybrids are
more variable than the hybrids themselves." ** But domesticated animals
and plants live under unnatural conditions, and they are therefore more
proline of |;emmules than wild species ; and as the body of a male hybrid
IS a new thmg, the cells will be much more likely than those of the pure
parent to throw off gemmules. The £ict that vanation is due to the male
influence, and that the action upon the male parent of unnatural or
changed conditions results in the variability of the child, is well shown by
crossing the hybrid with the pure species ; for when the male hybrid is
crossed with a pure female, the children are much more variable than
those bom from a hybrid mother by a pure father." It cannot be said,
however, that the evidence is as yet sufficient to warrant these general
conclusions.
In successful hybridisation, three results are common :— {a) a blending
of the parental characters, (6) more or less exclusive expression of the
characters of one parent, and {c) a form quite unlike either parent. What
direction the new variation will take cannot be predicted, but in many
cases the result is a re-appearance of the characters of an ancestral form.
In some cases this may mean that latent characters which have for a lime
been unexpressed are permitted to develop ; in other cases it may mean
that a new permutation of qualities has independently reproduced an old
pailern or combination. Herr von Guaita found that if the Japanese
dancing mouse was crossed with an albino, the second generation consisted
of grey mice like the wild forms. (" Ber. Nat. Ges. E'reiburg," x., 1898,
PP- 3^7-332.)
Professor Cossar Ewart took a pure white fantail cock-pigeon, which in
colour had proved prepotent over a bhie pouter, and paired it with a cross
previously made between an " owl " and an "archangel," and the result
SEXUAL REPRODUCTION. 1 65
was a couple of fantail-owl-archangel crosses, one resembling the Shetland
rock-pigeon and the other the blue rock of India. Again, a smooth-coated
white rabbit, derived from an Angora and a smooth-coated white buck,
was mated with a smooth-coated, almost white doe (grand-daughter of a
Himalaya doe) ; and the result was that in a litter of three, one was the
image of the mother, a second was an Angora, like the paternal grand-
mother, and one was a Himalaya, like the maternal great-grandmother.
For further details "The Penycuik Experiments" (1899) should be con-
sulted.
The hybrid offspring often resembles one of its parents much more than
the other. Thus Standfuss found that in reciprocal crossing the male is able
to transmit the characters of its species in a higher d^ree than the female.
A reverse result is noted below.
The relative maturity of the two sex-elements has been shown by
Vernon^ to be of importance in the hybridisation of sea-urchins. The
characters of the onspring incline to be those of the species whose
elements were relatively the more mature when fertilisation occurred.
Standfuss has also noted that the freshly hatched larva often closely
resembles the female parent ; that with growth a resemblance to the male
parent gradually increases; and that tne final extent of approximation
towards the male parent depends on the relative phylogenetic age of the
two species, the older being able to transmit its characters, whether of
structure or habit, Ix^tter than the young.
In pairing normal forms with varieties and local races, Standfuss found
(i) that when the norm {*^Gruttdart**) is crossed with a gradually formed
local race of the same species, the result is a series of intermediate forms ;
but (2) that when the norm is crossed with a sporadic variety, the result in
many cases is that the issue agrees either with the norm or with the sport,
intermediate forms being absent. (" Handbuch der paliiarktischen Gross-
Schmetterlinge," 2nd ed., Jena, 1896.)
In short, the result seems to depend on the issue of what may be called
the germinal struggle between hereditary characters of varying strength.
The results reached by Standfuss are not altogether corroborated by
other workers. Thus J. W. Tutt gives an account ("Trans. Entomological
Society, London," 1898, pp. 17-42) of experiments made by Riding and
Bacot in hybridising two allied species of Tephrosia — T, bistortata Goeze
{crepuicularia auct.) and T, crepuscularia Hb. (biundularia auct.). The
hybrids show all degrees between full fertility and complete sterility; they
may be fertile inter se and with the parent stock ; the phylogenetically older
species is more dominant in stamping its characters on the progeny and the
female parent more than the male ; a recently formed variet v may be pre-
potent over the type from which it has sprung. The re-crossmg of a hybrid
with one of the parent species produces offspring scarcely differing from that
parent species ; the inbreeding of the same cross produces a large percent-
age differing much from either parent form; the crossing of the hybrids
obtained from original reciprocal crosses tends to produce a mixed progeny,
some referable to known forms of the crossed species, others quite unlike
anything ever obtained in nature.
Henri Gadeau de Kerville calls attention to an interesting conclusion —
requiring, however, to be more carefully substantiated — that the results
of successful hybridisation arc much oftener males than females, and
that male offspring are more numerous in proportion to the specific distance
l66 THE EVOLUTION OP SEX.
between the two parents ("Bull. Sue. Zool. France," xxiv., 1899,
PP- 49-51).
The early researches of Kolreuter (1761) gave a firm basis to the study
of hybridisation among plants. The comparative easiness of experiment
has advanced the botanical side of the subject to far greater certainty than
the zoological conclusions can pretend to. Amon^ plants, as we should
expect from their greater vegetativeness, the fertility of hybrids seems
frequently established. Knight, Gartner, Herbert, Wichura, and others,
have brought together a great number of reliable observations, and
the whole subject has been admirably discussed by Nageli. For a
copious resume of the general results, for the most part after Nageli, the
student may be referred to chap. vi. of Sachs' Text-book of Botany, while
Wallace's " Darwinism " should be consulted for its rediscussion of hybri-
disation in animals.
§ 8. Telegony. — The belief has been for long current among breeders that
the effective impregnation of a female may influence not only the immediate
offspring, but subsequent offspring by a different sire. It is said that a
pure-bred bitch lined by a mongrel dog is thereby s]X)iled for future breed-
ing. The supposed influence is technically called telegony, and the supposed
facts are usually explained by supposing that the surplus sperms in the first
impr^nation exert an influence on the immature ova in the ovary, or by
supposing (in mammals) that the mother is aflected by her first offspring
through the medium of the placenta. The first point, however, is to make
sure that theie are facts to be explained, and this seems very doubtful.
What has been regarded as the influence of the first sire on the offspring of
the same mother by a second sire may receive some other explanation, may
be, for instance, an independent variation, or may be simply a revei-sion.
Much, if not most, of the alleged evidence is anecdotal; and careful
experiments made by Professor Cossar Ewart, on the lines of Lord Morton's
famous case, have yielded no evidence of the reality of tel^ony.
Dr Otto vom Rath has suggested (** Biol. Centralbl.,*' xviii., 1898, pp.
637-642) that the occurrence of badly-bred pups in a litter, which breeders
regard as the result of telegony, may be accounted for by co-infcetation, in
which case different sires are supposed to fertilise the ova which may be
shed into the oviduct at intervals of several days. But this, again, requires
further proof.
StXUAL ftEt»ROt)UCTIOjf. 1 67
SUMMARY.
1. Reproduction is but more or less discontinuous growth.
2. Sexual reproduction normally implies {a) s(>ecial reproductive cells,
distinct from the body; {d) the dimorphism of these cells; {c) theii
physiological dependence, — the ovum being unproductive without thf[
spermatozoon, and vice vers^.
3. The discoveries of Camerarius, Amici, Kolreuter, Sprengel, and
others, laid the foundations of our knowledge of sexual reproduction
in plants.
4. The history of research on fertilisation in animals well illustrates the
gradually increasing precision of scientific inquiry.
5. The conjugation processes seen in Protozoa are of much importance
in suggesting the origin of differentiated fertilisation.
6. The origin of fertilisation may be traced through the following
grades: — (a) plasmodial union, {d) multiple conjugation, {c) ordinary
conjugation, (d) union of dimorphic cells, {e) fertilisation of ovum by
spermatozoon.
7 Both in plants and animals hybridisation is often successful, but the
offspring frequently tend to be sterile. This, however, must not be
exaggerated.
sT Telegony has not been demonstrated.
LITERATURE.
See the already noted works of Balfour, Van Ikneden, Carnoy, Geddcs,
Iladdon, Hensen, Hertwig, M'Kendrick, Sachs, Vines, and Wilson.
For recent papers see Zoological Record, from 1886; and Journal of
Royal Microscopical Society.
For bibliography of hybridism in plants see M. Abbado in Nuovo
Giorn. Bot. Ital., V., 1898, pp. 76- 105, 265.303.
AcKERMANN, K. — Die Thierbastarde. Ber. Verein Kassel., 1897, pp.
103-121.
Born, G. — Beitrage zur Bastardirung zwischen den einheimischen Anuren-
arten. PflUger*s Archiv., XXXII., 1883, and Archiv. Mikr. Anat.,
XXVII., 1886.
EwART, J. C. — The Penycuik Experiments. 1899.
Hartog, M. — Some Problems of Reproduction. Quart. Journ. Micr.
Sci., 1891.
Johnson. — The Plastogamy of Actinosphserium. J. Morphol., IX., 189^.
Klebs, G. — Zur Physiologie der Fortpflanzung einiger Pilze. Jahrb. wiss.
Botanik, XXXIII., 1899.
Knuth, p. — Handbuch der Bliitenbiologie. 1898, 3 vols. See especially
Vol. I. for literature (from H. Mliller to 1898), and for general discus-
sion of pollination, cleistogamy, parthenogenesis, &c.
KoHLBRUGGK, J. F. H.— Der Atavismus. Utrecht, 1897, ji pp.
KoHLWRY, II. — Arten- und Rassenbildung. Eine Einfuhrung in das
Gebiet der Ticrzucht. Leipzig, 1897, 72 pp., 5 figs.
1 68 tkfe EVOLUTION OF SEX.
MOBius, M. — Beitrage zur Lehre von der f'ortpflanzung der Gewachse.
Jena, 1897, VI., and 212 pp., 36 figs.
Morgan, T. H.— The Development of ihe Frog*s Egg. New York,
1897, 192 pp., 51 figs, (with bibliography).
MuLLER, H. — Fertilisation of Flowers. Translation by D*Arcy W.
Thompson. London, 1883.
NussBAUM, M. — Beitrage zur Lehre von der Fortpflanzung. Arch. Mikr.
Anat, XLL, 1893, pp. 1 19-145.
Pflugbr, £. — Die Bastardzeugung bei den Batrachien. Pfliiger's
Archiv., XXIX., 1882; cf. Pfiuger and Smith, op, cit,, XXXIL,
1883.
Rbibmayer, Albert. — Inzucht und Vermischung beim Menschen.
Leipzig und Wien, 1897, 268 pp.
Reul. — Les Unions Consanguines ; Histoire de la Cr^tion des Races
C^^bres. Ann. M&i. V2terinaire, JXLVL, 1897, pp. i-S ei sea,
Rhumblbr. — Beitrage zur ^enntniss der Rhizopoden. Zeitsch. f. wiss.
Zool., LXL, 1895.
ScHAUDiNN, Fr. — Ueber die Copulation von Actinophrys sol. Sitzber.
Akad. Preuss. Berlin, 1896, pp. 83-89, 6 figs.
Standfuss, M. — Handbuch der palsearktischen Gross-Schmetterlinge
(2nd ed.}, Jena, 1896; cf. F. A. Dixey, Science Prc^ess, VII.,
1898, pp. 185-202.
Suchetrt, a. — Problemes hybridologiques. Joum. de PAnat. Physiol.,
XXXIII.. 1897, pp. 326.355.
Des Hybrides k I'elat sauvage. Tome I. Oiseaux, 8*. Paris,
CLIL, loOT pp., 1897.
VitRNON, H. M.— P. Roy. Soc. London, LXIII., 1898, pp. 228-231.
Verworn, M. — Biologische Protisten-Studien. Zeitschr. f. wiss. ZooL,
L., 1890.
Wager, H.— The Sexuality of the Fungi. Annals of Botany, XIII.,
1899, pp. 5^5-597-
Ward, M. — On the Sexuality of the Fungi. Quart. Journ. Micr. Science,
XXIV., 1884.
Wolters, M. — Die Conjugation und Sporenbildung bei Gregarinen. Arch.
Mikr. Anat., XXXVIL, 1891.
CHAPTER XII.
Pheory of Fertilisation.
''i"»
In his 49th Exercitation on the "efficient cause of the
chicken," Harvey thus quaintly expresses what has always
been, and still is, a baffling problem : — " Although it be a
known thing subscribed by all, that the foetus assumes its
original and birth from the male and female, and consequently
that the egge is produced by the cock and henne, and the
chicken out of the egge, yet neither the schools of physicians
nor Aristotle^s discerning brain have disclosed the manner
how the cock and its seed doth mint and coine the chicken
out of the egge."
§ I. Old Theories of Feriilisaiion. — (a) From Pythagoras
and Aristotle on to the " Ovists," of whom we have already
spoken, numerous naturalists have held the opinion that the
ovum was the all-important element, which only required to be
awakened to development by contact with the male fluid or
male elements. It must be allowed, that while ova may
exceptionally develop without sperms, the latter never come to
anything apart from ova.
(b) In contrast to the above opinion, we find ingenious
thinkers, so widely separate in time as Democritus and Para-
celsus, regarding the male fluid as very important, forestalling
Biiflbn and Darwin in fact in considering it in a sense an
extract or concentrated essence of the whole body. But it was
only after the spermatozoa were themselves detected that their
importance became unduly exaggerated, in the minds of those
who seem almost to have been nicknamed ** animalculists."
It seems probable enough that Leeuwenhoek himself (1677)
saw the spermatozoon entering the ovum, — he at least said
that he did, — but that did not prevent him from ascribing to
the male elements all the credit of development. This became,
as we have seen, a favourite hypothesis, and imagination sup-
plied more than modern magnifiers to those observers who
170 THJ£ EVOLUTION OF SEX.
detected in the spermatozoon the members and lineaments of
the future organism.
(c) The third opinion, that both elements are of essential
and inseparable import, is obviously alone consistent with the
facts. This view also has had its gradual development, only
one phase of which need be noticed. Even after the nature of
the spermatozoa as male-cells was recognised, that is to say,
even since the middle of the nineteenth century, an old con-
ception of the male influence lingered persistently. This
namely, that contact was not essential, but that a "sort of
contagion," a ** breath or miasma," "a plastical vertue,"
" without touching at all, unless through the sides of many
mediums," was sufficient to effect what we call fertilisation.
The above expressions are used by Harvey, who further says,
"this is agreed upon by universal consent, that all animals
whatever, which arise from male and female, are generated by
the coition of both sexes, and so begotten as it were per con-
fagium aliquodJ^ De Graaf attempted in vain to give more
precision to this "contagion" in his theory of an ^^ aura
seminalis^^ or seminal breath which passed from the male fluid
to the ovum. But the conception of an " aura " was only a
verbal cloak for that absence of definite knowledge which the
slow progress of observation still necessitated. The theory
was partly strengthened by a number of erroneous observa-
tions, which seemed to show that successful fertilisation could
occur when the genital passages of the female were apparently
blocked by malformation or disease. Spallanzani gave a death-
blow to the theory of an " aura," by showing experimentally
that contact of the male fluid with the ovum was absolutely
necessary. Even he, however, went away from the true con-
clusion, by maintaining that the fertile male fluid of toads was
destitute of spermatozoa. That the above vague conceptions
have been replaced by the certain conclusion, that intimate
cellular union is the sine qua non of fertilisation, we have
already emphasised.
§ 2» Modern Theories of Fertilisation — AforphohgicaL —
Recent investigators of the facts of fertilisation have generalised
their results in different ways according to their dominant bias.
Some mainly restrict themselves to stating the morphological
facts, and to emphasising the relative importance of cell-sub-
stance and of nuclei in the union ; others attack the deeper
problem of the physiological import of the process, — a problem
THEORY OF FERTILlSATlOfl. 171
the full solution of which is still remote ; while others have
confined themselves rather to discussing the uses of fertilisation
in relation to the species. Some representative positions on
each of these planes must be sketched ; and, first of all, the
more morphological theories, and the very important question
whether the union of nuclei is everything, or whether the union
of cell-substance has also its import.
(a) Hertwig^s K/>w.— Professor O. Herlwig, who was one of the first
carefully to follow out the details of fertilisation in animals, thus sums up
his ^*Theorie der Befruchtung''^ : — "In fertilisation, distinctly demon-
strable morphological processes occur. Of these the important and essen-
tial one is the union of two sexually differentiated cell-nuclei, the female
nucleus of the ovum and the male nucleus of the sperm. These contain
the fertilising nuclear substance, which is an organised substance, and acts
as such in the process. The female nuclear substance transmits the
characters of the mother, the male nucleus those of the father, to the
offspring.'* The nucleus is thus the essential element both in fertilisation
and in inheritance.
{b) Strasburget' 5 View. — What Hertwig maintains for animals, Stras-
burger does for plants. **The process of fertilisation depends upon the
union of the sperm nucleus with the nucleus of the egg-cell; the cell sub-
stance (cytoplasm) does not share in the process." '* The cell-substance of
the pollen-grain is only the vehicle to conduct the generative-nucleus to its
destination." It may become nutritive, he allows however, to the germ-
rudiment. *' Generally the uniting nuclei are almost perfectly alike,"
though there may be slight differences in the size of the nucleoli. " The
two cell-nuclei do not differ in their nature, they are not sexually differen-
tiated in the ways that the individuals are from which they originate. All
sex-differentiations only serve to bring together the two nuclei essential to
the sexual process."
The opmions of these two authorities are certainly representative, and
they both agree in emphasising that the nuclei arc all-important, and that it
does not matter much about the union of cell -substance. Some objections
to this view must be noticed, {a) It is permissible to doubt whether the
recent concentration of attention upon the nucleus has not led to some
under-appreciation of the general protoplasm. In the permanent conjuga-
tion of two cells, the entire contents of the two cells are obviously fused ;
and even when the union is temporary, Joseph has observed what looks
like an interchange of protoplasmic as well as of nuclear substance, {b)
There are a few observers still, such as Nussbaum, who maintain that in
fertilisation in animals the substance of the sperm is important as well as
its nucleus, {c) Strasburger notes the minimal quantity of cell -substance
so often present round the male nucleus, and urges that if it were im-
portant there would surely be more of it. But it is quite conceivable that
a minimal quantity of highly active protoplasm might have, like a ferment,
a momentous influence on a large quantity of a different character. (^ It
is, moreover, a very proliable view that cytoplasm and nucleus attain their
full significance only in inter-relation, forming what has been called a
** cell-firm." {e) Boveri made the delicate experiment of removing the
nucleus from a sea-urchin ovum which he afterwards fertilised. Although
172 THE EVOLUTION OF SEX.
the ovum had therefore only paternal nuclear material it developed into a
larva. More recently Delage has extended the experiments and has reared
normal larvae of sea-urchin, worm {Lanice\ and mollusc {Denialium) from
non-nucleated ovum-fragments which were successfully fertilised. He
describes this remarkable phenomenon under the title of merogony. (/)
The researches of Boveri and others show that the sperm brings with it
into the ovum a protoplasmic centre— a " cenlrosome " — ^which appears
to be of much importance in the preparation for division. In this
preparation, according to Boveri, the "muscular fibrils" of a special kind
of protoplasm (or archoplasm) literally move the nuclear elements.
" The movement of the elements is wholly the result of the contraction of
the attached fibrils, and the final arrangement of these nuclear elements in
the ' equatorial plate * is the result of the action of the archoplasmic
sphere exerted through the fibrils." Now this specially active protoplasm
has its centre in the two central corpuscles, each "ruling a sphere of
archoplasm." There seems general agreement as to the fact that the
sperm furnishes the centrosomcs in at least the majority of cases, and there
is no doubt that these minute bodies play a prominent part in the division
which follows fertilisation. At this stage, then, it seems rash to deny that
even the minimal cell-substance of the spermatozoon may, as well as its
nucleus, have a momentous influence in fertilisation.
§ 3. Physiological Theories of Fertilisation. — The morpho-
logical facts, established and verifiable by observation, form
the basis from which to attack the deeper problem of the
physiology of fertilisation. Here experiment is almost insuper-
ably difficult; only a few incidental results are as yet available;
the suggestions thrown out by various naturalists must there-
fore be appreciated according to their consistence with the
general principles of physiology, and with the general theory of
sex and reproduction. To some they may still appear a page
of probabilities.
Sachs compares the action of the male element upon the
egg-cell to that of a ferment. De Bary also suggests that pro-
found chemical differences exist between the two elements.
Very suggestive is the view of Rolph, who regarded the process
as essentially one of mutual digestion. His vivid words well
deserve quotation : —
"Conjugation occurs when nutrition is diminished, whether this be due
to want of light, or to the lowered temperature of autumn and winter, or
to a reduction of the organisms to minimal size. It is a necessity for
satisfaction, a gnawing hunger, which drives the animal to engulf its
neighbour, to *isophagy.* The process of conjugation is only a special
form of nutrition, which occurs on a reduction of the nutritive income, or
an increase of the nutritive needs, in consequence of the above-mentioned
conditions. It is an ' isophagy,' which occurs in place of ' heterophagy.'
The less nutritive, and therefore smaller, hungrier, and more mobile
organism we call the male, — the more nutritive and usually relatively more
THEORY OF FERTILISATION. 1 73
quiescent organism, the female. Therefore too is it, that the small starving
male seeks out the large well-nourished female for purposes of conjugation,
to which the latter, the larger and better nourished it is, is on its own
motive less inclined." Cienkowski has also inclmed to a similar view,
regarding conjugation as equivalent to rapid assimilation.
In Holotkuria iubuiosa and some other Echinoderms, N. Iwanzoff has
observed (*'Bull. Soc. Nat. Moscou," 1897, pp. 355-367, 1 pi.) that
immature ova, after a certain stage, show great sexual attraction for
spermatozoa, and emit many pseudopodia, while the mature ovum only
emits one. But the spermatozoa absorbed by the immature ova are
digested, which leads the author to the speculative suggestion that the
process of maturation weakens the nutritive vitality of the ovum so that it
is unable, fortunately, to digest the spermatozoon.
Simon also seeks to establish the following among other vague con-
clusions : — Sexuality has, he says, arisen twice (we should say much oftener),
once among plants, again among Protozoa. Two similar cells unite "in
order to reach the limit of their individuality." In both kingdoms the
union is at first protective, though in a different fashion in the two cases.
In the progressive dififerentiation, these two sex-cells are usually so con-
structed that the loss of substance in the union is reduced to a minimum,
hence the small inobile male and the large quiescent female cells. The
union brings about a chemico-physical process, which makes the female cell
capable of independent nutrition and growth, and evokes potential proper-
ties into actual life.
In marked contrast to Rolph's suggestion, and the view of
all those who believe that the sex-cells are profoundly different,
is the opinion maintained by Weismann. He denies that there
is a dynamical action in fertilisation. The momentous effect is
merely a restoration of the normal composition of the nucleus.
"The physiological values of sperm and egg-cell are equal;
they are as I : I. We can hardly ascribe to the body of the
ovum a higher import than that of being the common nutritive
basis for the two conjugating nuclei." The external differences
which are so obvious are only important as means towards the
conjugation of similar nuclei. " The germ-plasm in the male
and female reproductive cells is identical." Previous to the
essential moment of fertilisation, the germ-plasm is reduced in
maturation. Development will not take place unless the loss
be made good. This is what the sperm does in fertilisation.
In short, to Weismann the process is quantitative rather than
qualitative.
This supposition appears to us to be open to criticism, (i.) That the
nuclei are alone important in fertilisation, and that the cell substance is a
mere adjunct, cannot be said to be proved, and we have already noted some
of the facts which tell the other way. (2.) The structure of a cell is
recognised by all to be an expression of its dominant protoplasmic pro-
cesses. The sex-cells are usually highly dimorphic, and even Strasburger
174 THE EVOLUTION OF SEX.
allows that there may be minor differences in their nuclei, as well as the
marked divergence in their cell -substance. The nucleus cannot be
regarded as an isolated element, but as one which shares in the general
life of the cell. We have already interpreted the diflferentiated male and
female cells as respectively katabolic and anal)olic, and see no reason for
doubting, in spite of structural resemblance in the rough features of nuclei
(all that we know), that this difference saturates through the elements.
(3.) If the only important matter be the quantitative restoration of the
original amount of germ-plasma in the female nucleus, it seems difficult to
understand the phenomena of conjugation, whether permanent or transitory,
from which we believe fertilisation to have originated. (4.) The occasional
possibility of inducing division by replacing the sperms with other stimuli,
seems to point to a dynamical or chemical action. Thus Loeb induced
artificial parthenogenesis in sea-urchin ova by placing them for a couple of
hours in sea-water, to which some magnesium chloride had Iteen added.
(5. ) Delage*s evidence that non-nucleated portions of ova may be readily
fertilised and form normal larvsc, strongly suggests that the mingling of
heritable qualities, which is certainly one of the results of fertilisation, must
be distinguished from the physiological stimulus to division.
It is very desirable that the experiment which i^^ri has begim of
extracting a ferment (ovulase) from seminal matter and using it as a
fertilising stuff, should be confirmed or confuted.
It has been already noted, in regard to the origin of fertilisa-
tion, that the almost mechanical flowing together of exhausted
cells is connected by the stages of multiple conjugation with
the ordinary form of the latter, while the respective differentia-
tion of the two elements effects the transition to fertilisation
proper. Historically, then, fertilisation may be compared to
mutual digestion, and, though bound up with reproduction, it
may have arisen from a nutritive want. With the differentiation
of the elements on anabolic and katabolic lines, the nature of
the fertilising act becomes more definite. The essentially
katabolic male cell, getting rid of all accessory nutritive material
contained in the sperm-cap and the like, brings to the ovum a
supply of characteristic products which stimulate the latter to
division. The profound chemical differences, surmised by
some, are intelligible as the outcome of the predominant
anabolism and katabolism in the two elements. The union
of the two sets of products restores the normal balance and
rhythm of cellular life. At the same time, it is of course
certain that the spermatozoon is the bearer of the inheritable
paternal characteristics.
§4. Uses of I^ertUisaiion io the Species,— '^oi a few natu-
ralists have passed from the individual aspect of fertilisation to
its general import in relation to the life of the species. Why
THEORY OF FERTILISATION. 175
should fertilisation occur at all, if parthenogenesis in some
cases works so well ? Part of this question is almost illegitimate,
if the existence of male and female be, as we think, simply the
expression of a more developed swing of "the organic see-
saw" between anabolism and katabolism. The answers have,
however, much interest, and are valuable, so long as they are
not magnified so as to hide the deeper physiological problems
lying below. The origin and physiological import of fertilisa-
tion can never be explained by any elucidation of its subsequent
advantageousness.
The two naturalists who have recently reached the most
valuable results in regard to the results of fertilisation are
Maupas and Weismann. This they have done by very different
paths, — Maupas, in working out the details of conjugation in
infusorians ; Weismann, in his wider studies on the problems
of heredity and evolution. To Maupas, fertilisation is necessary
to prevent the death of the species ; to Weismann, fertilisation
is the ever-recurrent beginning of new vital changes, and the
continual preservation at the same time of the relative con-
stancy of the species. Several naturalists of the highest reputa-
tion have regarded fertilisation as a process which supplied a
fresh life-impulse to the species. Thus Galton has insisted,
with much clearness and force, on the liability of asexual, or
what he calls unisexual multiplication to end in degeneration
or extinction, and on the necessity of double parentage for
the preservation and progress of the species. Similarly, Van
Beneden, Biitschli, and Hensen have all spoken of the process
as a rejuvenescence ^rejeunissement, Verjungung). The asexual
process of cell-multiplication is limited ; conjugation in lower,
fertilisation in higher organisms supplies the recurrent impulse
which keeps the life of the species young. According to Van
Beneden, — **The faculty which cells possess of multiplying by
division is limited. There comes a time when they can divide
no further, unless they undergo rejuvenescence by fertilisation.
In animals and plants, the only cells capable of being re-
juvenesced are the eggs; the only cells capable of rejuvenescing
these are the sperms. All the other parts of the individual
are devoted to death. Fertilisation is the condition of the
continuity of life. Par elle le g^ndrateur ^chappe k la mort."
Hensen, in his admirable " Physiology of Reproduction," ex-
presses the same when he says: — **By normal fertilisation,
death is warded off (ferngehalten) from the germ and its
176 THE EVOLUTION OF SEX.
products.'' Biitschli has interpreted conjugation in similar
terms.
Weismann quotes the three opinions just mentioned, and
vigorously criticises them. He demands evidence for the
limitation of asexual reproduction assumed above, and speaks
of the "impossibility of proof." The whole "conception of
rejuvenescence has something indefinite and misty about it."
" How can one think that an infusorian, which by continued
division has at length exhausted its reproductive capacity, will
regain the same by uniting and fusing with another which has
also lost its power of further division ? Twice nothing cannot
give one; or if we assume that in each animal there persists
only half the reproductive capacity, so that the two together
would form one, this one can hardly call ' rejuvenescence.' It
would be simply an addition, as is under other circumstances
attained by simple growth, — that is, if we leave out of account
what in my eyes is the most important moment in conjugation,
viz., the mingling of two heredity-tendencies {Vererbungsten-
denzen),'^ He sarcastically compares the two exhausted
individuals to two exhausted rockets, which are supposed to
rejuvenesce in mutually affording the constituents of nitro-
glycerine. More forcibly he urges the difficulty suggested by
continued parthenogenesis, — a difficulty which we shall after-
wards have to discuss. " To the conception of rejuvenescence,"
he says, in conclusion, " I could only agree, if it were proved
that multiplication by division can never, — not merely in certain
conditions, — but never continue unlimitedly. This cannot,
however, be proved, just as little as the reverse." But Weis-
mann must surely admit that the demonstration of even some
cases where species, normally reproducing asexually, come to
an absolute standstill if conjugation be prevented, would give
considerable strength to the interpretation of fertilisation as
rejuvenescence. Maupas has given examples of such cases.
The French observer has shown, as we have seen, that a
process of fertilisation occurs in ciliated infusorians. By an
elaborate process of nuclear division, disruption, elimination,
interchange, union, and reconstruction, two "slipper animal-
cules " fertilise one another. What is the meaning of all this ?
Each infusorian, after conjugation, proceeds to divide, but
the results are to all appearance the same as it previously pro-
duced. There is no special sexually produced generation.
It has been often alleged that the subsequent dividing is
THEORY OF FERTILISATION. 1 77
accelerated by conjugation; but Maupas finds that this is not
so. The reverse in fact is true, — it is a loss of time. While a
pair of infusorians (Onychodtomus grandis) were indulging in a
single conjugation, another had become, by ordinary asexual
division, the ancestor of from forty thousand to fifty thousand
individuals.
Moreover, the intense internal change preparatory to fer-
tilisation, and the general inertia during subsequent reconstruc-
tion, not only involve loss of time, but expose the infusorians
to great risk. It seems then like a condition of danger and
death rather than of multiplication and birth.
The riddle was, in part at least, solved by a long series of
careful observations. In November 1885, M. Maupas isolated
an infusorian (Stylonichia pusiu!ata\ and observed its genera-
tions till March 1886. By that time there had been two
hundred and fifteen generations produced by ordinary division,
and since these lowly organisms do not conjugate with near
relatives, there had, of course, been no sexual union.
What was the result ? At the date referred to, the family
was observed to have exhausted itself The members, though
not exactly old, were being born old. The asexual division
came to a standstill, and the powers of nutrition were also
lost.
Meanwhile, however, several of the individuals, before the
generations had exhausted themselves, had been removed to
another basin, where they conjugated with unrelated forms of
the same species. One of these was again isolated, and
watched for five months. The usual number of successive
generations occurred; members removed at different stages
were again observed to conjugate successfully with unrelated
forms, and this was done on to the one hundred and thirtieth
generation. After that, however, the family being again near
its end, the removal was no longer any use. About the one
hundred and eightieth generation, the strange sight was seen
of individuals of the same family attempting to unite with
one another. The results were, however, «//, and the con-
jugates did not even recover from the effects of their forlorn
hope.
Without the normal sexual union, then, the family becomes
senile. Powers of nutrition, division, and conjugation with
unrelated forms, come to a standstill. The first symptom is
decrease in size, which may go on till the individuals only
12
1 78 THE EVOLUTION OF SEX.
measure* a quarter of their normal proportions. Various
internal structures then follow suit, *' until at last we get form-
less abortions, incapable of living and reproducing them-
selves." The nuclear changes are no less momentous. The
important para- or micro-nucleus may partially or completely
atrophy, and conjugation is thus fatally sterile. The larger
nucleus may also become affected, ** the chromatin gradually
disappearing altogether.'' Physiologically too, the organisms
become manifestly weaker, though there is what the author
calls a " surexcitation sexuelle." Such senile decay of the
individuals and of the isolated family inevitably ends in
death.
The general result is evident Sexual union in those infu-
sorians, dangerous perhaps for the individual life, — a loss of
time so far as immediate multiplication is concerned, — is in a
new sense necessary for the species. The life runs in cycles
of asexual division, which are strictly limited. Conjugation
with unrelated forms must occur, else the whole life ebbs.
Without it, the Protozoa, which some have called " immortal,"
die a natural death. Conjugation is the necessary condition
of their eternal youth and immortality. Even at this low level,
only through the fire of love can the phoenix of the species
renew its youth.
The need of extreme carefulness is shown, however, by the somewhat
discrepant results reached by D. Joukowsky (" Verb. Nat. Med. Vcrcin
Heidelberg," vi., 1S98, pp. 17-42). After five months' culture his colony
of Paramecium caudatum showed no nuclear degeneration, but only a
marked reduction of cilia and a consequent sluggishness. In P, puirinum
effective conjugation between the descendants of one individual was ob-
served, but the author admits the probability that this has its limits. In
another infusorian, PUurotricha /anceoiata, over 458 generations were
observed without the occurrence of degeneration, and Joukowsky supposes
that degeneration depends not on the number of generations merely, but
on the rapidity of their successive occurrence.
At the beginning of this century, the too-much-forgotten
biologist Treviranus directed attention to fertilisation as a
source of variation, and his suggestion has been several times
independently revised.
Thus Brooks, to whose works we have repeatedly referred,
has emphasised not only the importance of fertilisation as a
source of progressive change, but further, that the male ele-
ment is much the more important in this connection.
Similarly, though on somewhat different lines, Weismann
THEORY OF FERTILISATION. 1 79
suggested that the mingling of male and female germ-plasms
was a source of those variations on which natural selection
operates. " Sexual reproduction is well known to consist in
the fusion of two contrasted reproductive cells, or perhaps
even in the fusion of their nuclei alone. These reproductive
cells contain the germinal material or germ -plasm, and this
again, in its specific molecular structure, is the bearer of the
hereditary tendencies of the organisms from which the repro-
ductive cells originate. Thus in sexual reproduction, two
hereditary tendencies are in a sense intermingled In this
mingling, I see the cause of the hereditary individual char-
acteristics; and in the production of these characters, the task
of sexual reproduction. It has to supply the material for the
individual differences from which selection produces new
species."
Few will be inclined to oppose the thesis that sexual
union is productive of variation. To discuss the relations
of this view to other theories of variation is beyond our
present scope. Thus Weismann has suggested that germinal
variations may also arise because of the reducing divisions
which precede fertilisation or amphimixis, as he calls it, and
furthermore through nutritive and other stimuli which operate
through the body upon the germ-plasm. We should also
mention Hatschek's suggestion that sexual reproduction is a
remedy against the operation of injurious variations. We can
readily imagine that the excess of some particular line of
anabolic or katabolic differentiation may be neutralised through
fertilisation, and it may be in this way that the pairing of
diseased individuals is sometimes mercifully condoned by
nature.
Mobius may be cited here for his vigorous protest against
the prevalent idea that continuous vegetative multiplication
necessarily results in degeneration. He instances such cases
as the banana and date palm, which are never reproduced
sexually in cultivation, but it must be remembered that arti-
ficial selection is here in operation to sustain the standard.
He admits, however, the advantages of sexual reproduction
both in conserving the specific characters and in prompting
new variations. (** Beitrage zur Lehre von der Fortpflanzung
der Gewachse." Jena, 1897.)
In regard to close in-breeding we have still much need of
experiment, but it seems fairly certain that with a healthy
l8o THE EVOLUTION OF SEX.
Stock this may go far without disadvantageous results. It
seems rather to fix and strengthen desirable characteristics.
On the other hand, if the stock be markedly tainted, the in-
breeding is continued at the risk of degeneracy.
Even when the stock is sound to start with, there seem to
be limits to close in-breeding, as Vom Rath found in experi-
ments with rats, and Von Guinta in experiments with mice. As
many breeders have recorded, there is a tendency to debility,
abnormality, and sterility.
According to Reibmayr, the success of a human race is in
part dependent on the alternation of periods of sustained in-
breeding, which serve to " fix " racial characters, and periods
of crossbreeding in which the advantages of *' fresh blood "
are secured.
THEORY OF FERTILISATION. l8l
SUMMARV.
1. Old theories of ''ovists/' " anitnalculistSi'' and of the ''aura
seminalis. "
2. Modern morphological theories incline to lay the whole emphasis
upon the nuclei. The conclusions of Hertwig and Strasburger are strongly
in favour of this view. The import of the centrosomes and general proto-
plasm must not, however, be overlooked. Several facts suggest that the
all-importance of the nuclei has been exaggerated.
3. Modem physiological theories of fertilisation are necessarily very
tentative. Sachs compares it to fermentation; Rolf to mutual digestion.
To Weismann, the process appears quantitative rather than qualitative.
Suggestion that the male cell brings to the ovum a supply of characteristic
chemical substances which act as a stimulus.
4. Uses of fertilisation to the species. Many regard fertilisation as a
necessary rejuvenescence of the life of the species. Weismann criticises
thb view, but his criticism must be read in the light of the researches of
Maupas, who has shown that without conjugation the members of an
isolated family of infusorians eventually cease to feed and divide, passing
through stages of degeneration and senility to extinction. In this case,
conjugation is essential to the continued vitality of the species. According
to brooks and Weismann, fertilisation is an important source of variation.
LITERATURE.
Baillet. — Quelques mots sur les croi:>ements dits au premier sang.
Mem. Ac. Toulouse, V., 1893.
Bos, J. RiTZEMA. — Untersuchungen ttber die Folgen dcr Zucht in engster
Blutverwandtschaft. Biol. Centralbl., XIV., 1894, pp. 75-Si-
BovERl, Th. — Befruchtung. Anatomische Ergebnisse, I., 1^93, pp. 386-
485 ; and subsequent reports in the same Record.
CONKLIN, E. G.—The Fertilisation of the Ovum. Wood's HoU Bio-
logical Lectures, II., 1894, pp. 15-35, 10 figs.
Hertwig, O. — Das Problem dcr Befruchtung, &c.; Jenaische Zeitschrift
fllr Naturwissenschaiten, XVIII., 1885.
Klebs, G. — Ueber das Verhaltniss des mannlichen und weiblichen
Geschlechts in der Natur. Jena, 1894, 30 pp. ,
KcEHLBR, R. — Pourquoi resemblons-nous ^ nos parents ? Elude Physio-
logique sur la fecondation, sa nature et son origine. Revue Philoso-
phique. XXXV., 1893, pn. 337-386, 38 figs.
Maupas, E.— Comptes Rendus, 1886, 1887; and Archives ile Zoologie
experimentale, 1888.
Rkgnault, F. — Des efifets de I.1 consanguinite'. Rev. Scient., LL, 1893,
pp. 232-6, 266-71.
Kmumblek, L. — Zellleib, — Schalen- und Kern-Verschmelzungen bei den
Khizopoden und deren wahrscheinliche Beziehungen zu phylogene-
tischen Vorstufen der Metazoenbefruchtung. Biol. Centralblat. ,
XVIIL, 1898, pp. 2\-2(i ei seq.
1 82 THE EVOLUTION OF SEX.
SCHIMKEWITSCH, W. — Ziir Frage uber die Inzestzucht. Biol. Central-
blatt, XVI., 1896, pp. 177-181.
Standfuss, M. — Handbuch der palaearctischen Gross- Schmetterlinge,
2nd ed. Jena, 1895, 392 pp., 8 pis.
Strasburger, E. — Ncue Untersuchungen uber den Befruchtungsvorgang
bei den Phanerogamen, als Grundlage fur eine Theorie der Zeugung.
Jena, 1884.
Wilson, E. B. — The Cell in Development and Inheritance. New York
and London, 1896, 371 pp., 142 figs.
Weismann, a. — Opp, cit.^ especially Die Redcutung der sexuellen Fort-
pflanzung fur die Selektions-Theorie. Jena, 1886.
CHAPTER XIII.
Degenerate Sexual Reproduction or Parthenogenesis.
§ I. History of Discovery, — From very early times there appears
to have been an impression, that in exceptional circumstances
reproduction might occur without fertilisation. Even Aristotle
gave reasons for believing that, without sexual union, the un-
fertilised eggs of the honey-bee might give rise to perfect adults.
We now know that he was right, in his conclusion at least, so
far as the development of drones is concerned. In the early
belief in Lucina sine concubitu^ much that was erroneous was
intermixed with a prevision of the truth; nor could we expect at
an early date that asexual multiplication (/>., apart from ova alto-
gether) would be kept distinct from what we now mean by
parthenogenesis, or the development of ova without union with
sperms. In 1701, Albrecht observed that a female silkmoth,
which had been isolated in a glass case, laid fertile eggs ; and
though this was for long discredited, the occasional partheno-
genesis of this insect has been repeatedly confirmed by com-
petent observers.
In 1 745, the ingenious Bonnet drew attention to what is
now a very familiar fact, the successive generations of virgin
plant-lice or Aphides. Throughout the summer, he observed
the production of numerous generations of these little insects,
all females, necessarily therefore all virgins, and yet fertile. So
strange did the fact appear, that it was for long utterly dis-
credited. Reaumur eluded the difficulty, by affirming that the
Aphides were hermaphrodite ; but Dufour soon proved that this
was erroneous, though he could only confess his ignorance in
referring the phenomena to "spontaneous or equivocal gen-
eration," in which "the act of impregnation was in no degree
concerned." The facts, however, were repeatedly re-observed.
Kirby and Spence admitted them as incontestable, but could
regard them only as " one of the mysteries of the Creator, that
human intellect cannot fully penetrate."
184 'i""E EVOLUTION OF StX.
Meanwhile Schiiffer had observed the occurrence of par-
thenogenesis in minute aquatic crustaceans, the study of which
has since shed some vivid light on the whole subject. Pastor
Dzierzon had also clipped the wings of queen-bees, and in thus
preventing their nuptial flight and impregnation, observed
that the eggs they laid developed only into drones. The facts
soon began to be recognised, extended, and thought over by
naturalists of the standing of Owen (1843), ^^n Siebold (1856),
and Leuckart (1858), whose conclusions have afforded a firm
basis for the abundant subsequent observation and speculation
on this interesting subject.
§ 2. Degrees of Parihenogenesis, — If we start then \vith Von
Siebold's definition of parthenogenesis, as the power possessed
by certain female animals of producing offspring without sexual
union with a male, it will clear the ground to notice, in the first
place, the numerous different degrees in which this develop-
ment without fertilisation may occur.
(a) Ariificial Farihcfiogcnesis, —Thexe are a few curious
observations which go to show that in exceptional circum-
stances ova may develop when the male stimulus is replaced
by some artificial reagent. These observations must still be
taken cum grano sa/is^ but they may be. at least suggestive of
further experiment. Dewitz observed unfertilised frog ova to
undergo segmentation (sic) in corrosive sublimate solution. In
some cases one division occurred, in others several; in some
cases irregularly, in others normally. It happened both when
the ova were left in the reagent, and when they were merely
dipped and returned to water. The eggs experimented on
were those, of the two common frogs Rana fusca and R.
esculentq^ and of the tree-frog (f/yla arborea). But it must be
noted that Leuckart long ago noted the occurrence of spon-
taneous division in frog ova. Similarly, TichomirofT, experi-
menting with the unfertilised ova of the silkmoth, which are
occasionally parthenogcnctic, was surprised to observe that ova,
which would not of themselves develop parthenogcnetically,
might be induced to do so by certain stimuli. These con-
sisted in rubbing the unfertilised ova with a brush, or in
dipping them for two minutes in sulphuric acid and then
washing them. In both cases, he says, a percentage of the
ova thus artificially stimulated developed. It must be remem-
bered that occasional parthenogenesis occurs in this insect,
and all that Tichomiroff did was to incite this. J. P^rez notes
Regenerate sexual kEPkODUCtioN. 185
("P. V. Soc. Sci. Bordeaux," 18967, pp. 9-10) that gentle
friction of silkmoth eggs stimulates parthenogenetic develop-
ment, but that it occurs apart from this, especially in the eggs
of very robust females. There is no doubt that reagents may
considerably modify ova; thus the brothers Hertwig showed
Iiow it was in this way possible to overcome the non-receptivity
of the ovum to more than one sperm. Nor can one forget how
sexual reproduction in parasitic fungi tends to disappear, being
possibly replaced by the stimulus aflTorded from the waste
products of the host. In a similar way, the multiplication of
cells, so frequently associated in disease with the presence of
bacteria, has been referred by more than one pathologist to the
** spermatic influence" of these micro-organisms, or of the
katastates which they form. The most circumstantial account
of successful artificial parthenogenesis is that given by Professor
Jacques Loeb, who reared larvae of a sea-urchin from unferti-
lised eggs which had been left for a couple of hours in sea-
water plus a solution of magnesium chloride, and then returned
to the normal medium. Balfour has also cited a remarkable
observation of Greeff, who saw unfertilised ova of the common
starfish developing in ordinary sea-water, in a perfectly normal
fashion, only more slowly than usual.
(d) Pathological Parthenogenesis, — It has very occasionally
been noticed in higher animals, where true parthenogenesis is
wholly unknown, that an unfertilised egg starts off on its own
resources without any male stimulus whatever. This is noted
by Leuckart for frog ova, by Oellacher for hens' eggs, and by
Bischoff and Hensen even in mammals.
Barfurth has re-investigated the question of parthenogenesis in the hen*s
egg, a^d finds that the segments are non-nucleated, therefore not triie cells.
He explains the process as wholly physico-chemical. (Versuche uber die
parthenogenetische Furchung des Hiihnereies, ''Arch. Entwickelungsme-
chanik.,'* vol. ii., 1896.) In the ovarian ova of various mammals, e,g,
guinea-pig, and rabbit, Janosik has observed (i) regular formation of polar
bodirs, (2) division into numerous nucleated segments either equal or un-
equal, (3) fragmentation into m<iny minufe parts. (Die Atrophic der
FoUikel und ein seltsames Verhalten der Eizelle, "Arch. f. Mikr. Ana-
toniie," xlviii., 1896, pp. 169-181, i pi.)
Such cases must be regarded as rare abnormalities, com-
parable perhaps to pathological formations which not unfre-
quently take place in the ovary, and it is hardly necessary to
say that in no case did the development proceed far.
l86 THE EVOLUTION OF SEX.
(c) Occasional Parthefiogenesis, — In some of the lower
animals, which are not themselves normally parthenogenetic,
but have relatives which are, occasional parthenogenesis has
been frequently observed. These differ from the above cases,
since the results are more successful, often in fact reaching
maturity, and also in this, that since related forms are partheno-
genetic, the ** abnormality " is evidently of a much milder type.
The common silkmoth is a good example of this occasional
parthenogenesis, which certainly occurs, though rare both in
the genus and family. Out of 1,102 unfertilised eggs of the
silkmoth, observed by Nussbaum, only 22 developed, and that
only up to a certain point "A whole series of insects,"
Weismann says, " reproduce exceptionally by parthenogenesis,
for instance many butterflies, but that never to the extent that
all the eggs which an unfertilised female lays develop, but
only a fraction, and usually a very small fraction of the total
number, the rest perishing. Examples of successful occasional
parthenogenesis (to the extent at least of producing males) are
furnished by those worker bees, wasps, and ants which excep-
tionally become fertile."
(^ Partial Parthenogenesis, — The queen-bee, as has been
already mentioned, is impregnated by a drone in her nuptial
flight. The sperms thus received are stored up, and used to
fertilise the eggs as she lays them in the cells. Not all the eggs,
however, but only those which will produce future queens or
else workers. Other eggs, to all appearance similar, are un-
fertilised, and these, as Dzierzon first clearly showed, develop
solely into drones. It is a well-established fact that if ferti-
lisation be prevented by the imperfect development of the
wings, or by clipping them, the queen only lays drone eggs.
The same happens when she is old and her store of male
elements exhausted, or when the sperm receptacle has been
removed. Von Siebold carefully examined the eggs from drone-
cells, and found that they never contained spermatozoa.
Hensen notes an interesting side fact, obviously corroboratory,
that "German queen-bees, fertilised by Italian or Cyprian
drones, produced hybrid females but pure drones, a proof that
on the latter the sperm does not operate." Again, it some-
times happens that what are called ** fertile workers " crop up,
which in consequence of some accident or misdirected inten-
tion in the nutrition, become less abortive than the host of
semi-females which make up the body of workers. They are
DEGENERATE SEXUAL REPRODUCTION. 187
fertile enough to lay eggs, but their female organs do not seem
to admit of their being impregnated. Certain it is that they
only produce drones. What has just been said in regard to
bees, is also true of some wasps and ants.
(e) Seasonal Farthenogefiesis, — In some of the minute
aquatic crustaceans (Cladocera), popularly included under the
general title of water-fleas, parthenogenesis only occurs for a
season, and is periodically interrupted by the birth of males,
and the occurrence of the ordinary sexual reproduction. Males
generally reappear in the disadvantageous conditions of autumn,
but Weismann denies that there is a direct connection between
these facts. The common Aphides are parthenogenetic for
a succession of generations, sometimes as many as fourteen,
throughout the summer, but the cold and hard times of autumn
bring back the males and the sexual process. The fertilised
egg lives on through the winter, and develops with the warmth
of the next spring. By keeping up the temperature and
nutritive optimum for three or four years in the artificial
summer of a glass case, Reaumur and Kyber succeeded in
rearing as many as fifty continuous parthenogenetic genera-
tions. In the gall-wasps (Cynipidae) there is usually only one
parthenogenetic generation between the normal sexual repro-
ductions, but in many insects besides Aphides there are several.
It ought to be noted that the parthenogenetic Aphides are
hardly at the same structural level as the females which are
fertilised ; but as the differences mainly lie in the absence of
certain accessory genital organs, there is no reason for regarding
the parthenogenetic forms, as some have done, as larval
(/) Juvenile Parthenogenesis, — Cases do occur, however,
where larval forms become precociously reproductive (as some-
times happens among higher organisms), and produce offspring
parthenogenetically. Such precocious production pf partheno-
genetic ova must be distinguished from the entirely asexual
reproduction exhibited by many larvae. No very firm line can
indeed be drawn, but in the last cases no cells which can be
called ova are present. In 1865 Professor N. Wagner observed
what has been much studied since, that in the larvae of some
two-winged or dipterous midges (^.^., Miasior\ the cells of the
reproductive rudiment develop into larvae within the mother-
larva's body. The mother falls a victim to her precocity, for the
brood of seven to ten larvae literally feed upon her to the death.
They finally leave the corpse and begin life for themselves,
l88 THE EVOLUTION OF SEX.
only, however, themselves to fall victims to a similar fate. The
process may thus go on for several generations, during which
the ova, or pseudova as some would insist upon calling them,
become smaller and smaller. Eventually the larvae become
too constitutionally poor to be precociously parthenogenetic,
and develop into adult midges — male and female, the latter
producing, however, only a few eggs.
In another dipterous insect known as ChironomuSy the ova
begin to be produced at a very early stage, are laid just at the
time when the larval life ends, and develop parthenogenetically.
According to Jaworowski, by the rupture of the ovarian mem-
brane the ova fall into the body-cavity, where the abundant
nutritive stimulus takes the place of fertilisation. Juvenile
parthenogenesis is also said by Von Siebold to occur among
the Strepsiptera, little insects which infest bees.
(£) Total Parthenogenesis, — Lastly, in some of the minute
aquatic crustaceans and in many rotifers no males have ever
been found. There is every probability that the partheno-
genesis is thus total ; and as the numbers are abundant, it has
apparently been established without detriment to, at least, the
continuance of the species.
§ 3. Occurrence of Far t/unoge nest's, — In three distinct sets
of animals — rotifers, crustaceans, and insects — parthenogenesis,
has become a confirmed physiological habit.
{a) Take first the curious liltle rotifers, or wheel-animalcules, which
alx}und both ia fresh and salt water. They are usually placed in the
chaotic alliance of worm-types, and have long been famous for their alleged
power of surviving prolonged desiccation. With one or two exceptions
the males are markedly different from the females, and are usually small
and degenerate. In one group (Philodinadzc) the females have two ovaries,
while males have never been found. They have dwindled out of existence.
In the rest the females have one ovary, part of which has degenerated into
a yolk-gland, and small males occur. These are quite superfluous as mates,
however, for parthenogenesis prevails. Even when impregnation, which is
a peculiarly random process, occurs, the sperms appear to miss their mark,
and to perish in the lx)dy-cavity. The numbers keep up, notwithstanding,
so that we have here an entire chvss where parthenogenesis has firmly
established itself.
[h) Among crustaceans, parthenogenesis is restricted to the lower orders,
viz., branchiopods and ostracods. In the former, it is exhibited by the
brine-shrimp Artcmia and the common fresh- water A pus in one division;
by daphnids [e.g.^ Daphnia and Moitta, common •'water-fleas") in the
other. In ostracods, some species of the common Cypris are partheno*
genetic. If a female water-flea, say Daphnia^ be isolated from birth,
she becomes the mother of an abundant progeny of females. Males and
DEGENERATE SEXUAL REPROtillCTIOV. 1P9
Mxual reproduction do, however, eventatUy return, &nd the nme ii
probably true of the majority. Among three thousand specimens of the
oririe-slurimp only one male occurred ; nhile Von Siebold repeatedly in-
vestigated every member of a colony of Afiis, once over five thousand in
number, without linding a single male. At other limes he found one per
ceot., while in certain unknown conditions
(probably when food is scarce and life generally
Dnfiivourable) the males may be developed in
In the daphnidi, which have been so success-
fntly studied by Weismann, ibe fads are more
complex. There ire two kinds of ^gs — winter
and summer ova. The former are large, thick
shelled, canble of resisting drought and the
like, and of remaining long latent. They only
develop if fertilised, and always pcodnee females.
In every way they are highly anabolic ova.
The summer eggs, on the other hand, are
smaller, and thinner in the shell. They can
develop without fertilisation, and that a indeed
in some cases physically impossible. Males
are produced from summer eggs alone. They
usually appear in autumn, when life is becoming
harder, or the conditions more katabolic
In the little cyprids the reproductive rela-
tions ate very varied. Thus in Cyfris ovum and
Nolodnmut monaekut the males are abundant
all the year round, and parthenogenesis is
unknown. In other species, e.g., Candoita
Candida, the males ate still frequent, but
parthenogenesis nevertheless occurs. Lastly,
parthenogenesis prevails i
a rertiliscd egg^t
We may find instances of permanent, o^f^'
seasonal, or even merely local parthem^enesia, lii
—a variety of occurrence which strongly sug- 1"
i ni porlan ce. g™"i<alFyTo'i' 'bl»d'," and
{{) In insects, as we have seen, the degrees w on itrougb a FucctMion
of parthenogenesis are very varied; so too_ is Ae^^^'f™k"f'^
the systematic position of the forms in which luppeu. anil luuil it-
normal parthenc^enesis occurs. Two butter- production ttiunii. Atihe
flies {Psycht helix and So/enaiia, 2 sp.) and a '"I* "5 eulier appwr.
Iwetle [Gai/mfiAysa), some coccus -in sects and J^rawed"™" """ '*
Aphide<i, certain saw-flies (Tenihrediuidse) and
(;all-wasps (Cynipidx), are normally parlhenogenelic In the butterflies
just noticed, the males seem to disappear for a sttetch of years, and the
species gets on without them. The male of Psyiht helix is very rare, and
was for long unknown. When the males are developed in SoletioMa
trinquelrella, it is interesting to notice that they may predominate in
numliers over the females. A whole brood may lie male ; tney are brought
190 THB EVOLUTION OF SEX.
back with a nish. About a score of molhs, i Deluding the silkmoth {Bombyx
moH) and death's-head {Sphinx atropos)^ have been Known to exhibit casual
parthenogenesis; but the beetle above noticed stands alone. Bassett, Adler,
and others, have demonstrated an interesting alternation of parthenogenesis
and ordinary sexual reproduction in numerous gall-wasps. Forms which
had been regarded as quite distinct, and had received diflcrent generic
titles, have been shown in about a score of cases to be merely the partheno-
genetic and normal forms of the same insects. From a winter gall the
parthenogenetic form emerges which produces a summer gall. In this a
sexual form is produced, which eventually gives rise to the winter gall.
§ 4. Parthenogenesis in Plants, — The passive bias is so strong in plants,
that it is easy to understand the rarity of parthenogenesis. The ^[g-cell
which develops of itself must retain the stimulus which the male element
in other cases supplies. It is natural, then, that what predominates in the
active rotifers should be uncommon in the sleeping plants. In some of the
flowering plants, what looked like parthenogenesis has repeatedly been
described, especially in regard to a native of New Holland, known as
Ccelebosyne. When cultivated in Europe, the male flowers degenerate, and
according to Braun and Hanstein disappear. Yet fertile seeds are pro-
duced. Karsten found, however, that stamens often persisted; while
Strasburger has shown that what developed were not true egg-cells, but
adventitious growths from cells outside the embryo-sac The same is true
of some other cases. Dr A. Ernst has recently described what he calls
true parthenogenesis in a Menisperm found by him in Caracas, and named
Disciphania Ernst ii. *' Female plants, which bore no male flowers, and
which were grown perfectly isolated where there was no possibility of the
access of pollen from another plant, produced in three successive years an
increasing number of fertile fruits."
Kerner von Marilaun and H. O. Juel have shown that in Antennaria
aJpitta fertile seeds are produced without impregnation, the male plants
being very rare and producing no functional pollen - grains (*'Bot.
Centralbl.," Ixxiv., 1898, pp. 369-372), and there are several similar cases
on record.
In the lower plants, however, cases of parthenogenesis abound,
apparently as one of the stages in the degeneration of sexual reproduction.
It has been casually observed of a species of the stonewort {Chard), that
when grown in certain waters the male organs disappear, yet the plants
continue multiplying. More interesting are the Fungi. To illustrate sexual
degeneration, De Bary gives a series from Fungi like those which kill the
salmon and potato (Saprolegniaeand Peronosporeae). What happens first,
is the degeneration of the male organs. The katabolic sex from beginning
to end is the more unstable. The male function goes first, but the form
remains after the reality has ceased. After a while, that is in related
species, the form goes too. Sometimes the function is changed, and the
male organs become protective sheaths. De Bary's series may be briefly
summed up.
(i.) In Pythium, the male organ discharges most of its protoplasm into
the female, — the usual process.
(2. ) In Phytophthora, only a very small portion is thus given, and we
may almost say asked, for there are curious demand and supply
nrrcingements and compulsions between the male and female organs
in these Fungi.
DEGENERATE SEXUAL REPRODUCTION. I91
(3.) In Peronospora^ there is no perceptible passage of protoplasm from
male to female, though, without going back to the *'aura
seminalis," we may allow the possibility of subtle osmosis.
(4. ) In some Saprolegnioe, there are indeed the usual antheridia or male
organs, which are directed towards the female organs, but do not
open. The ** explosive" character is diminishing.
(5.) In others, the male organs never get near the female.
(6.) In others, there are no male organs at all, but the female cells
develop as usual.
Parthenogenesis is thus reached, as the result plainly of a degenerative
process. We can follow the story further, however, forestalling for the
moment the subject of the next chapter. The male organ has degenerated,
we have seen, while the female organ holds on its course. But this is
not always so; in many cases it follows suit, and asexual reproduction
remains.
Now why should these Fungi among plants exhibit numerous instances
of parthenogenesis? The more intimate the parasitism, the more degene-
rate the sexual reproduction, and all trace of it is often lost. It may be
that in the vital economy of the species the nutritive stimulus of the host
in some measure takes the place of fertilisation. Or it may be that the
stimuli which Klebs has shown to be operative in inducing the occurrence
of asexual or sexual reproduction in Algae and Fungi which have both modes
of multiplication, are absent in the parasitic forms above referred to.
Male parthenogenesis, paradoxical as it sounds, is really exhibited
among lowly Algae. That is to say, a small spore (or male-cell) which
normdly unites with a larger and more quiescent one (or female-cell), may
occasionally start developing on its own resources. The result, however,
is poor enough. As those spores are on the border line between asexuality
and differentiated sex-elements, the retention of a vegetative power of
division even by the incipient male-cell is not surprising. Nor must it be
forgotten that the mother-sperm-cell itself has a power of parthenogenetic
development. It divides, like its homologue the ovum, into a ball of cells,
but, having none of the conservative coherence of the latter, breaks up
into spermatozoa. It is exactly comparable to the interesting Protozoon
(Magosphara) which Haeckel saw, which did its best to get beyond the
Protozoa, but failed as soon as it had succeeded. A single infusorian-like
cell divided into a ball of cells, but the ball had no coherence and broke up
into infusorians once more.
§ 5. The Offspring- of Parthenogenesis, — The fate of parthenogenetic
ova is very diverse. They may all perish, or all succeed ; they may turn
out wholly males or wholly females. Hensen notes the following suggestive
series, with decreasing reproductive, as opposed to constitutional, energy
at each level : —
(i.) Hermaphrodites, then only females.
(2.) Series of females, then mixed brood.
(3.) Several females, mixed brood, then only males.
(4.) Series of mixed broods, then males, or death of ova.
(5.) Mixed brood, with much mortality.
(6 ) Males only.
(7.) Development only for a few stages.
Rolph has a ditterent arrangement, but the same idea: —
(I.) Exceptional parthenogenesis with uncertain result {e.g,^ Silkmoth).
193
THE EVOLUTION OF SEX.
(2.) Normal, producing males only (female solely from fertilised ova)
{e.^.t B^s).
(3.) Mostly males, with occasional females (^.^., Nematus),
(4.) Mostly females, with exceptional or ^leriodic males (^.^., Afms^
Artemia).
(5.) Only female, males unknown (^.^., many Kutifers).
That parthenogenetic ova should develop with such diverse results is
not at all surprismg. The absence of fertilisation removes one of the
factors determining sex ; but food, temperature, age of ovum, &c., remain,
and produce bias now to one side, now to the other. To this we shall
presently return; meanwhile the facts of offspring may Ix; more clearly
expressed thus : —
Result.
D
>
o
H
H
M
o
y,
X
H
<
^Nil
Partial and j^athological development
Great mortality in a mixed broo<l. .
^ *s alone
(J 's mostly, a few 9 's .
<J 's and 9 's (one generation)
6 *s, and more than a few 9 's
9 9 9 (a succession), then a pretlomin-
ance of <J 's
9 9 9, then equal numbers of <5 *s and 9 's
9 9 9, then a minority of ^*s among 9 's
9999, very rare <J 's . . .
9 9 9 9, non-functional i 's among 9 's
^ 9 9 9 9 , ad infinitum, no <J *s
Example.
Most organisms.
Rarities mentioned.
Many insects.
Hive-Ixre and some
other forms.
Nemaius (allied to
l)ce).
Most gall-wasps.
Some saw-flies.
Some water-fleas.
Soltnobia sometime*;.
Aphides ; some water-
fleas.
Many water-fleas.
Most rotifers.
Many rotifers.
§ 6. Effects of Parthenogenesis, — Since parthenogenesis is
dominant in rotifers, and well established among water-fleas
and plant-lice, it is very plain that whatever else it affects, it is
anything but prejudicial to numbers. An aphis will continue
for days producing a viviparous brood, at the rate of one per
hour; the offspring soon begin themselves to multiply; and
Huxley calculates that if this continued for a year without
mortality, a single aphis would be the ancestor of a progeny
which would weigh down five hundred millions of stout men !
Not gardeners only have cause for gratitude that climate and
enemies prevent such untoward increase. But there are other
desiderata besides numbers. Can it be said that partheno-
genesis favours the general life and progress of the species?
More than one of the old naturalist*!, and in recent years
Brooks, Gal ton, Weismann, and others, have laid emphasis on
the value of fertilisation as a fountain of change. To Weismann
DEGENERATE SEXUAL REPRODUCTIOX. 1 93
the intermingling of the male and female "germ-plasms" in
fertilisation is one of the possible sources of variation. If it be
removed therefore, as in rotifers, the species will be so much
the less likely to progress, the establishment of parthenogenesis
will be a handicapping of evolution.
But Weismann has shown that variation still occurs in
the parthenogenetic Cyprids, and variations of Rotifers, e.g.,
Anuraa cochlearis, have been often recorded. In the well-
known case just mentioned, however, the varieties are cor-
related with the seasons, and it seems to us probable that they
are seasonal "modifications" rather than true constitutional
variations.*
While recognising, then, the occurrence of variations in
forms where the reproduction is parthenogenetic, or even quite
asexual, we suggest the probability that the absence of fertilisa-
tion involves some diminution in the frequency and range of
variability.
§ 7. Peculiarily of the Parthenogenetic Ova. — Before a theory
of parthenogenesis is sought, the natural question arises, Are
these eggs that develop of themselves in any way peculiar?
{a) For a while it was supposed (^^., by Balfour) that
parthenogenetic ova did not form polar globules, and the
theory based upon this regarded the retention of these bodies
as taking the place of fertilisation. The demonstrated occur-
rence of one polar globule in several parthenogenetic eggs
partially demolished this theory, and it is only within the last
two or three years that it has been restated in accurate form.
(b) Simon shrewdly points out, that in some of the most
marked cases of parthenogenesis the sex-cells are insulated
from the body at a very early stage. Thjs is notably so in
those midges which reproduce parthenogenetically even before
maturity. It is certainly striking that these forms should
unite an extreme earliness in the embryonic separation of
the germ-cells with a most precocious reproduction. These
germ-cells are ova which have a much less circuitous history
than in most cases ; they have far fewer cell-divisions behind
them, they have thus a reserve power of division which other
ova have not ; they are able, in fact, to develop of themselves.
Although this is not known to be true of all instances of
normal parthenogenesis (^.^., rotifers), it is true of some, and
that to a greater extent than was known when Simon wrote.
On the other hand, some forms where parthenogenesis is
13
194 THE EVOLUTION OF SEX.
unknown {e.g.y leeches and Sagitfa\ also exhibit the same early
differentiation of germ-cells, so that we can only look upon
the fact as one of the auxiliaries of parthenogenesis
{c) The peculiarity of parthenogenetic ova, which has of
late attracted much attention, is that they extrude only one
polar cell, — not two, like other eggs. Weismann, with the
assistance of Ischikawa, proved this for the parthenogenetic ova
of Rotifers and Ostracods, such as Polyphemus^ Lepiodora hya-
iina, Sida crystallina^ Cypris reptans, Blochmann has also
corroborated Weismann's discovery by observations on aphides,
but he found two polar bodies in those unfertilised eggs of
bees which give rise to drones. The occurrence of two polar
bodies was also noted by Platner in the parthenogenetic ova
of a butterfly {Liparis), The apparent contradiction has been
in part explained by the discovery of Brauer that in the par-
thenogenetic ova of the brine-shrimp Artemia^ two modes of
maturation occur. In most cases only one polar body is
formed, in other cases two are formed, but the second does not
leave the egg and behaves just like a sperm-nucleus. A further
enquiry into the exact history of the nuclear rods or chromo-
somes shows that the two cases can be plausibly reconciled.
§ 8. Theory of Parthenogenesis, — (a) We may begin with
Balfour's view of the case^ though that of Minot has the priority.
** The function of forming polar cells has been acquired by the
ovum for the express purpose of preventing parthenogenesis."
If they were not formed, parthenogenesis would normally occur.
This is expressed in curiously teleological language, but the
main idea is clear enough, — the retained polar cells replace the
sperm nucleus.
iji) " In accordance with Minot's hypothesis of sexuality, it
might be assumed that in parthenogenetic ova the male element
was retained, and that the cell remained a true asexual cell, and
did not become a sexual element." '* Blochmann and Weis-
mann have shown that this is the case, by their discovery that
in parthenogenetic ova only one polar globule is formed, while
there are always two in ova which are impregnated; hence it
is probable that one polar globule (by hypothesis, male) is
retained."
Minot's words are not beyond criticism either, though they
are not teleological An ovum which retains a male element
is not happily described as remaining asexual; it would be
better to call it a case of intra-cellular hermaphroditism. It is
DEGENERATE 3EXUAL REPRODUCTION. 1 95
more important, however, to notice how Minot cleverly adapts
his theory to increased knowledge of the facts. The partheno-
genetic ovum only retains one polar globule, — one male element
is enough; two would be "polyspermy," which is abhorred.
(c) There was no fear that Rolph would indulge in teleology,
rigid necessitarian as he was. Parthenogenesis of ova was to
him the more natural process, the sperm a subsequent impor-
tation. " There is for the ovum a certain minimal mass, which
must be surpassed if it is to develop at all; and a second mini-
mum, which the ovum must attain, if a female is to be produced."
Abundant nutrition of the ovum tends to parthenogenesis, pro-
ducing male offspring, as the lower stage; but if the second
limit be attained, resulting in females. In the opposite direc-
tion, if the ovum has fewer resources, it requires to be fertilised.
Females or males will again result according to the state of the
elements. If no fertilisation occur, the dependent ovum must
of course die. Rolph is always suggestive, but he erred in
regarding the sex-elements too quantitatively, in missing the
qualitative antithesis of sex, and the opposition observed in
cell-division.
{d) Strasburger also lays emphasis, in a subtler and more
technical way, on nutritive conditions. " In the rare cases of
parthenogenesis, specially favourable nutritive conditions may
counteract the lack of nuclear plasma." He notes three
different ways in which this may happen, and also inclines to
believe that retention of polar globules would favour partheno-
genetic development. It is important to notice how two
naturalists, so very different in their manner of attacking a
subject as Rolph and Strasburger are, come to this conclusion
at least in common, that favourable nutritive conditions favour
parthenogenesis. All the cells in the body tend to multiply,
the ova retaining this power develop embryos.
(e) Weismann has a peculiar right to be heard on the nature
of parthenogenesis; for not only has he been for many years
an investigator of the tiny daphnids or water-fleas, but he made
the important discovery, already noticed, that parthenogenetic
ova extrude only one polar globule. There has not been time
yet to prove that this is always true, but the probabilities are
strong that it is. Weismann*s first supposition was that in
maturation of ordinary ova the first polar division got rid of
" oogenetic *' nuclear plasm, and that the second removed part
of the essential germ-plasm; and that in the maturation of par-
196 THE EVOLUTION OF SEX.
thenogenetic ova only the first process occurred This view is
expressed in the following paragraphs.
" Parthenogenesis occurs when the entire sum of the ancestral elements
persists in the nucleus of the ovum. Development by fertilisation demands,
however, that half of these ancestral elements must first be extruded from
the ovum, whereupon the remaining half, in uniting with the sperm nucleus,
regains the original number.
" In both cases the l^eginning of development depends upon the presence
of a definite, and indeed similar mass of germ-plasma. In the ovum which
requires fertilisation, this is afforded by the importation of the sperm-
nucleus, and development follows on the heels of fertilisation. The par-
thenogenetic ovum already contains the necessary mass of germ -plasma, and
this Incomes active as soon as the single polar body has freed the ovum
from the oogenetic nuclear-plasma."
He has now given up the hypothesis that the first polar body removes
'* oc^netic idioplasm," and the second germ-plasm; he admits that germ-
plasm is got rid of in both cases. In parthenogenetic ova, however, where
only one polar body is extruded, a sufficient quantity of germ- plasm is
retained to make development without fertilisation possible.
The view which Weismann now holds {vide ** The Germ-Plasm," 1893)
seems to amount to this, that the reduction of germ-plasm in the matura-
tion of ordinary ova is too great to admit of independent development, but
that in parthenogenetic ova the reduction is less and development without
fertilisation occurs. It may be urged that we have not as yet sufficiently secure
data in regard to the reducing processes in parthenogenetic ova, and it must
be noted that there are some authorities who refuse to grant that the em-
phasis which Weismann and others have laid on the reducing divisions is
justified. Thus Professor Hartog maintains ( " Natural Science, xiii. , 1898,
pp. 115-120) that the process of nuclear reduction does not affect the
quantity of nuclear material, but only the number of segments into which
it is divided. He suggests that it is the achromatic plasma or linin which
bears the hereditary characters, and that the chromatin has only a mechanical
function.
Weismann*s pre-occupation with questions of inheritance has given a
bias to his theor}', making it morphological rather than physiological A
given quantum of germ-plasma, he says, fits the ovum to develop. The
parthenogenetic 6vum keeps enough. The ordinary ovum extrudes so
much that it has to get it back again from another source. It appears to us
more in accordance with the facts to suppose that the entrance of the sperm
has a twofold importance, — {a) It bears with it certain hereditary charac-
teristics, doubtless in the nucleus for the most part ; {d) it brings with it a
stimulus to division of a qualitative character, doubtless in some part in its
small cell-substance. This last function — the dynamic function — Weismann
wholly denies. The sperm has to him only a quantitative function.
(/) Our theory of parthenogenesis may now be stated. Just
as the spores which illustrate the beginnings of sex may some-
times dispense with conjugation and germinate independently,
so may ova develop parthenogenetically. These are to be
regarded as incompletely diflferentiated female cells, which
DEGENERATE SEXUAL REPRODUCTION. 1 97
retain a measure of katabolic (relatively male) products, and
thus do not need fertilisation. Such a successful balance
between anabolism and katabolism is indeed the ideal of all
organic life. In parasitic fungi, sexual reproduction disappears,
and surrounding waste products presumably help the purpose
otherwise effected by sexual organs, so peculiarities in the con-
ditions of parthenogenetic ova may explain the retention of the
normal balance which makes division possible without the
usual stimulus of fertilisation. Abundant and at the same
time stimulating nutrition (Rolph), early differentiation of the
sex-cells (Simon), the general preponderance of reproductive
over vegetative constitution (Hensen), their liberation before
the anabolic bias has carried them too far, are among these
{parthenogenesis (f).
sex(s>.
disease (D).
f parthenogenesis (p)
sex(s).
disease (D).
Male < sex (s).
I
Diagram illustrating the theory of parthenogenesis.
favouring conditions. Artificial parthenogenesis can be in-
duced in some cases by chemical reagents; thus in Loeb's
experiments with sea-urchin ova magnesium chloride seemed
to have (perhaps very indirectly) the same physiological role as
spermatozoa. The incipient segmentation observed in a few
ova is an independent effort to save themselves from being too
big to live, since they are not passive enough to remain dor-
mant. Waste has set in, self-digestion begins, the cell is forced
into the expedient of division. In higher animals this is all in
vain : in lower animals such imperfectly differentiated female
cells are commoner; they form the parthenogenetic ova.
§ 9- Origin of Parthenogenesis. — From the occurrence of
parthenogenesis in the animal series, it is certain that it has
198 THE EVOLUTION OF SEX.
originated as a degeneration from the ordinary sexual process.
It is no direct persistence of a primitive ideal state, though in
a sense a return to it.
It seems to us misleading to interpret the occurrence of
parthenogenesis as due to "motives" and "important ad-
vantages." These are afterthoughts of our importation. It is
not easy indeed to keep from metaphorical language which
suggests that polar globule-formation is a " contrivance," and
parthenogenesis a " device." Such casual words are of little
account ; but it is going far to say, as Weismann does, " that
sexual reproduction has here been given up, not by any
chance nor from internal conditions, but from quite definite
external grounds of utility (Zweckmassigkeitsgrunden)." Our
position is the converse, that parthenogenesis arises because of
necessary internal conditions, and may be perpetuated because
of certain advantages.
DEGENERATE SEXUAL REPRODUCTIOJ^. I99
SUMMARY.
1. Parthenogenesis was formerly believed to be of wider occurrence
than it really is, but it is definitely known to be not uncommon in lower
animals.
2. Artificial, pathological, occasional, partial, seasonal, juvenile, and
total parthenogenesis must be distinguished.
3. The occurrence of parthenogenesis is especially weH seen in rotifers,
crustaceans, and insects.
4. It is rare among plants, but certainly occurs in some.
5. The offspring of parthenogenetic ova are very diverse.
6. The effects of parthenogenesis on the species deserve consideration,
especially by those wno find in sexual intermingling an important source of
specific variation.
7. So far as we know, parthenogenetic ova (with two or three excep-
tions) form only one polar body.
8. Parthenogenetic ova are here regarded as imperfectly differentiated
female cells, retaining certain characteristics which compensate for the
absence of fertilisation.
9. In origin parthenogenesis is regarded as a degeneration from the
ordinary sexual process.
LITERATURE.
See especially the already cited works of Balfour, Brooks, Hensen,
Minot, Rolph, Sachs, Weismann; also — *
Blochmann — Ueber die Richtungskorper bei Insekteneiern. Biolog. Ccn-
tralblatt, VII., and Morpholog. Jahrbuch, XII.
Brooks, W. K. — Law of Heredity. Baltimore, 1883.
Gkrst.€:cker. — Bronn*s Klassen und Ordnungen des Thierreich, Vol. V.,
Arthropoda.
GlARi), A. — Sur le d^veloppement parth^nog^n^tique de la microgam^te
des metazoaires. C. R. Soc Biol. Paris, 1899, 4 pp.
Hudson and Gossr. — The Rotifera. London, 1886.
Karsten, H. — Parthenogenesis und Generations- Wechsel im Thier und
Pflanzenreiche. Berlin, 1888.
Leuckart. — Art. "Zeugung" in Wagner's Handworterbuch d. Physiol.,
Bd. IV., 1853.
Owen. — Parthenogenesis; or, The Successive Production of Procreating
Individuals from a Single Ovum. London, 1849.
Plate. — Beitrage zur Naturgeschichte der Rotatorien. Jenaische Zeitschft.
f. Naturwiss, XIX., 1886.
Seidlitz, G. — Die Parihenogenesis. Leipzig, 1872.
Von Siebold. — Beitrage zur Parthenogenesis. Leipzig, 187 1.
Simon, F.— Die Sexualitat, &c., Inaug. Dissertation. Breslau, 1883.
Weismann, A. — Beitr. zur Naturgeschichte der Daphnoiden. Leipzig,
1876-79. Ueber die Zahl der Richtungskorper und tlber ihre Be-
deutung fur die Vererbung. Jena, 1887. The Germ- Plasm, 1893.
Weismann, A., and Ischikawa, C— Ber. naturforsch. Ges., Freiburg,
IIL, 1887.
Wilson, E. B. — The Cell in Development and Inheritance, 1896.
CHAPTER XIV.
Asexual Reproduction.
g I. Artificial />tt/«w«. —Weeping willows are by no means
scarce trees in Britain, yet, as they never flower, they must all
have grown from slips. In other words, their multiplication
is asexual and artificial. So too, only more naturally, the
Canadian pond-weed (Anacharis) has spread prodigiously in
our lochs, canals, and rivers, never bearing male flowers, but
owing its increase wholly to the asexual process. Every one
knows how the gardener increases his stock by slips and
cuttings, thus taking advantage of the power a part has to
reproduce the whole. Bananas, potatoes, and yams are in the
same way propagated asexually, and this is also the usual
REPRODUCTION. iOI
method in the case of olives, figs, and date palms. Quite in
the same way, cultivators or bath sponges are able to bed out
little fragments. In the last century, the Abb^ Trembley
delighted himself and others by the now familiar observation,
that lo get many hydra polypes, the simplest and quickest way
was to cut one in pieces. A fragment will reproduce the
whole, provided always that it have to start with fair samples
of the different kinds of cells in the body, and that it be not
loo minute. The same may be done with the much larger
sea-anemones. So the earthworm, curtailed by the spade, does
not necessarily suffer loss, though it may suffer pain. The head
portion grows a new tail, and even a decapitated portion may
Tilt Fotmaiion of a Sponge Colony (Ol^nliui) hy
reproduce a head and brain, not that this is saying much for
these.
§ 2. RegtneraHon. — Spades and knives are not exactly
instruments of nature, but they have their counterparts. Fight-
ing with a rival a crab may lose its claw, or the same may
happen in the frequently fatal moulting. Slowly, however, the
loss is made good; the cells of the stump multiply, and arrange
themselves in obedience to the same necessities as before, and
a limb is regenerated. Many an appendage among the lower
animals is from time to time nipped off, only to be grown
again. A snail has been known to regenerate an amputated
eye-beaiing horn twenty times. A starfish readily surrenders
an arm, and a lizard its tail. Indeed, many animals may be
said to have learnt organically, though not consciously, that
20i THE EVOLUtlON OP SEX.
be lost. For animals, like men, are often wiser than they wot
of. In the panic of capture, strong convulsions may occur ;
thus the sea-cucumber may surprise and perhaps shock its
persecutor by the ejection of its viscera ; or a tetanic contrac-
tion of the muscles makes the slow-worm brittle in the hands
of its captor. The power of regeneration is most marked in
echinoderms, but persists as high up as reptiles. The re-growth
of part of a lizard's leg is the chef-d^osuvre in this line. Beyond
that, regeneration is restricted to little things. We constantly
regenerate the skin of our lips, but we cannot naturally re-
place an amputated limb.
The regenerative capacity depends on primary properties of
the living matter and of the organism — which we are far from
understanding, -^but it seems probable, as Rdaumur, Lessona,
Darwin, Weismann, and others have pointed out, that its dis-
tribution and mode of occurrence are adaptive, being related
to the normal risks of life. According to Lessona's law, which
Weismann has elaborated, regeneration occurs in those organ-
isms and in those parts of organisms which are in the course
of nature most liable to injury. (See Weismann, "Natural
Science," xiv., 1899, pp. 305-328; " Anat Anzciger," xv., 1899,
PP- 445-474.)
This position may be argued for in two ways. One may
try to show that all parts known to be markedly capable of
regeneration are liable to be lost or injured under natural
conditions ; and one may show that parts not very liable to be
injured are not regenerated, although they are often of great
importance.
It must be admitted that there are cases of regeneration —
e.g,^ of a stork's bill or a newt's eye — where the natural liability
to injury is not at first sight evident. But inquiry shows that
male storks fight savagely, and that the head, and possibly the
eye of the newt, are often attacked by the larvae of the water-
beetle {Dyliscus mar^inaiis). But an explanation of the re-
generative capacity of the protected abdominal limbs of
hermit-crabs, which T. H. Morgan has demonstrated, is more
difficult, unless one supposes that the power has been inherited
from ancestral Crustaceans whose abdomen was unprotected,
or that the liability to injury to which the supposed adaptation
is related is associated with the process of moulting. That
the latter supposition should be tested is made clear by the
ASEXUAL REPRODUCTION. idj
But Morgan's criticism of Weismann*s position has not yet
been adequately met.
As to the second point, it will be admitted by most that
injuries to internal organs are seldom made good.
§ 3. Degrees of Asexual Reproduction, — The keynote of the
subject was struck by Spencer and Haeckel, when they defmed
asexual reproduction as discontinuous growth. All growth is a
reproduction of the protoplasm and its nuclear elements, or, in
short, of the cells ; all reproduction (excluding the important
event of fertilisation) is growth. The ovum, asexually produced
from the parent ovum or its lineally descendant cells, grows and
reproduces itself in turn, building up the embryo. The embryo
grows into an adult organism, and the surplus of continued
growing energy results in the asexual production of buds, or
the sexual discharge of differentiated reproductive elements.
We may start from the ordinary processes of cell-multiplication
and regeneration exhibited in the normal organism. Then
come the processes by which lost members are regenerated,
involving more or less serious extra growth. From these we
may pass to the rarer and yet not rare cases, where the artificial
halves or fractions of an organism can grow into wholes.
Normal and frequent, however, are the very abundant cases of
budding, where a sponge or hydra, zoophyte or coral, has
surplus enough to grow off new individuals, which remain con-
tinuous with itself. The parent organism, whether zoophyte
or strawberry-plant, has an asexually produced progeny round
about, and in asexual continuity with itself. But they do not
always remain continuous ; the hydra produces buds, but
eventually sets them adrift. This is still better seen in many
of the hydroids, where individuals are separated off as swim-
ming-bells or medusoids. The multiplication has become
discontinuous. Continue the process, and we find the libera-
tion of special cells, clinging often for a time to the parent,
generally dependent for development on union with similar
cells of complementary constitution; we find, in fact, the sexual
reproduction which, in the higher organisms, so thoroughly
replaces the asexual process.
§ 4. Occurrence of Asexual Reproduction in Plants and
Animals, — In plants, as one would expect from their vegetative
constitution, the asexual process is common, particularly among
the lower forms. The common liverworts {Marchaniia and
Zunularia\ through the formation of asexual buds or gemmae
a04 THE EVOLUTION OF SEX.
in the cups upon their thallus, rapidly overrun our flower-pots,
and become a pest of the greenhouse. Many ferns too, notably
among the Aspleniums, reproduce by bulbils, arising upon the
frond ; and the bulbils which arise in the axils of the leaves of
the tiger-lily are familiar missiles for every child accustomed
to a flower-garden (see figs. pp. a 1 1 and 240). The Alliums,
and some of our common grasses also, furnish us with examples
of the replacement of flowers by separable buds. Asexual re-
production, or multiplication by more or less discontinuous
growth, without the differentiation of special and mutually
dependent sex-cells, occurs from the simplest animals on to the
tunicates or sea-squirls, from the base lo just over the line
which separates backboneless and backboned animals. It is
necessary, however, to review the groups.
Preloma. — Fertiluation began in almost mechanicnl fusion. Reproduc-
tion begin; wilh almost mech.inical rupture. The unit mass of protoplasm,
becoming loo big for control, breaks. Thus it saves itself, and at the same
tiin« multiplies. Such biealiage may be seen in some primitive amceboid
ASEXUAL REPRODUCTION. 20$
fonns, but it also occurs in a few of the relatively high infusorians. That
the breakage sometimes means dissolution is certain ; nor is reproduction
ever so very far removed from death.
The rupture becomes orderly and systematic in budding. This may l)e
multiple, as in the common Arcelia^ where a number of small buds are
constricted off all round. But the process is oftener concentrated in one
extrusion or overflow. In budding, the separated daughter-cell is in vary-
ing degree smaller than the parent, and the process resembles an overflow.
When the bud is approximately equal to the parent, and the process is of
the nature of a constriction, it is, of course, division.
The division may also be multiple, taking place in rapid succession and
in limited space, ft.^., within a cyst. Then we speak of spore- formation.
The last three modes of multiplication are exceedingly common among
Protozoa.
These buddings and divisions are not, of course, rough and ready pro-
cesses. The nucleus almost always shares in them in an orderjy and com-
plicated fashion. There are variations in its behaviour as in higher
animals, but there is no doubt that cell-division, with a gradient of pro-
gress like ever3^hing else, is essentially one and the same in the vast
majority of cases. Gruber has been especially successful in proving that
fragments of Protozoa, artificially separated without nuclear elements,
cannot live long. Though they may retain for a time contractility and
irritability, they cannot ^d or grow. The nucleus is essential to life,
though sometimes it seems to disappear, and become as it were a difluse
precipitate in the protoplasm.
Sponges, — In sponges no one can fail to recognise the impossibility of
drawmg any rigid line between growth and asexual reproduction. Between
simple extension of the parent mass, and the budding off of new indi-
viduals, no sure distinction can in many cases be made out. Sponges do
not divide, though they may be cut up, yet some give ofl" discontinuous buds.
An outgrown tube may lose connection with the parent, or a great tumour-
like mass may be slowly extruded, or tiny brood-buds may be set adrift to
shift for themselves. In disadvantageous conditions the surface of a sponge
sometimes gathers into minute superficial buds, by means of which it is
possible that the life is saved.
In the fresh-water sponges, in disadvantageous circumstances, — of cold
in some countries, heat and drought in others, — some of the cells club
together to form gemmules, which often save the life of the otherwise
dying sponge. They are complex enough, with sheaths and spicules, and
sometimes even with a float, but in principle they simply do by a multiple
union what is otherwise attained by ovum and sperm. Best known in this
respect is the freshwater sponge {S^ngilla); but gemmules have also been
described in other common sponges, e.g.^ in Cliona^ the liorer in oyster
shells.
CoeUnterates. — In such names as zoophytes, sea-flrs, sea-roses, there
is a suggestion of the undoubtedly plant-like character of many of the
ccelenterates. A sessile habit is very general, though often only a phase
in the life-history, and asexual reproduction runs riot. A well-fed hydra
is proHBc in bud-bearing; and numerous gradations connect this with the
myriad colonies exhibited by many hydroids. The individuals forming a
united family share in the common life and nutriment. As the colony
becomes complex, it is often physically impossible for all the members to
306 THE EVOLVTION OF SEX.
renuun on terms of even appronimale equality of internal and eiteinal
conditions. One becomes relatively overfed, another staired. Slight
differences of function gradiiall)' Iwcome emiihluised and exagfrerated, till
division of labour is eilablished. The slruclural aspect of this is difleren-
tialion or polymorphism among the members of the colony, and resnltt in
the establishment of nuttilive and reproductive, sensitive and protective,
"persons." Thus in the commnn Hydrailinia, Che open-mouthed nutri-
tive individuals ate markedly conlraaied with the dependent leproduelive
persons; and again, in dilTi^rent form, the rhythm repeats itself In the
contrast between active, offensive, and sensitive elongated membeia, and
entirely passive and abortive spines, which form a chevaux-de-frise under
xhellet of which the others cower. It is usually supposed that the sessile
hydroids are in a sense degenerate from more active ancestral types. The
free-swimming embryo becomes exhausted, settles down, and exnihita pre-
dominant vegelativeness with postponed sexuality. In many cases, bow-
of i]h ncuidl a lice {pfycif^afui t
Itsdfdu
ever, there is a recovery of the ancestral liberty of action, for modified
*' persons " are set adrift as active, free-swimming, sexual medusoids.
There are, however, active forms of tlie true medusoid type (Trachy-
medusse) which never descend to the sessile nadir of existence, but yet
exhibit the asexual tendency of the class in forming temporary clusters of
pendent huds. Lang has described a temoikalile compound medusoid
{Gailrailatla raffatlii), which has sometimes as many as nine stomachs,
and may be assumed lo he highly nutritive. The remarkable point, how-
ever, is that the compound adult is the result not only of continued bud-
ding, but of a process of rectangular incomplete division. Along with
someothetsitleadson towards the Portuguese man-of-war, or aphonophore
series. Here the larva develops at first into a simple medusa-like indi-
vidual, hut this buds off a manifold series of " pcrson<i," which, by
dislocation or even migration, become arranged in all the beauty oX the
siphonophore colonies, which surpass even Mydraelittia in their division of
labour. It is difficult enough in some cases to distbguish between Irae,
ASEXUAL REPRODUCTION. SO?
" persona" — vvliich Ilaeckel calls " Medusomes" — and mere organs like
protective tnncts, vrhich are also Inidded off.
In another direcliun, viz., among the true jelly-lishes (Acraspeda),
where an aclive habit greatly preponderates, we stil! nnd the occurrence of
asexual multiplication. Some forms- (i(.^., Filasia) are entiiely free; at
the opposite extreme a few (Liiceniai I'a) may be described fls sedentnrT;
between the.ie we find the ■•- — '■*- "■'■:-'■ --'■'— J"~- i- ...
tTom ta.ag, Hflei Hueckcl.
There remain two classes of ccelenlerates, — the Ctenophora, liltc BirBt,
wliich represent a climax of activity, and never divide; and (he Actinotoa
(sea-anemones and corals), which lead lo a passive* terminus again, and
exhibit profuse asexual multiplication. A few sea-anemones divide nor-
mally, just IS ihey may be mulilplied by ariilicia] cutting. Fragments may
also lie given off in an «rl>itrary sort of fashion, remindmg one of the o- —
" ■ ' ' ■■ - . ■ - udiedbyH. RToi
'. PP- 345-360. ' Pl-).
ao8 THE EVOtUXrON OF SEX.
Ihereis a eieat variety of asexual miiltiplicnlion, which may occur by lonEi-
tudinal fissiiin. Inceralion. and budding. The budding of coraU takes mniiy
lama, resulting in the quaint compiexily of brain-ciiials and ihe liUe. In
one sea-nnemone {Gmiattinia fvelifeia), where transverse division occurs,
il is interesting la notice (hat this has only been observed in young forms
with undeveioped seiual organ;. It recalled, in fact, the asexual multipb'
cation of a young jelly-fish. In another of the corals (one of the Aniipa-
iharin) Brook observed that a nutritive "person" may, by constriction,
form a reproductive individual on either side.
H'ariiis. — The lower worm-types are roughly distinguishable from most
of the higher by ibe broad fact that they are all of a piece, without rings
or segments. 'A physiological link, however, between
*> worms of only one segment and those with many, is
. found in the asexual chains which some of the former
occasionally develop. Thas the little lurbellarian
^ d A/icroilomum linean may bud off ■ temporary chain
of sixteen individual links. The budding b^ns at
b the posterior end, and what is partly separated off is a
portion in excess of the normal size. The second link
grows till it attains the usual adult site, and as it rx-
jj ceeds this forms a. third link. At the same lime Ihe
original individual may also be doing the same, and
thus a chain of four is formed. Two more buddings
by each of the links bring the asexual process to a
climax, and then the individuals separate from one
another and liecome sexual in freedom. It is important
]ll lo notice that the asexual reproduction Jakes place in
favourable nutritive conditions, and as each individual
^ exceeds its normal limit of growth. In some allied
planarians Ihe asexual mullipTicntion is effected not by
budding but by division. Zacharias observed that when
j^ nutrition was checked the vegetative increase ceased,
and sexual reproduction set in. Notquile parallel with
the above, but quite asexual, is the prolific multiplica-
tion characteristic of the llukes and tapeworms. The
common live(-fluke has several asexual generations
DiagramBuilie repre- before il finds its final host in the sheep, ond Is sur-
mJikii rf ■ chwil P"*^'' '" ^•'■s respect by some of its relatives. A
ofindividiulsinthg bladder-worm, in pas^ve ease, with a plethora of
Turbcllarian worm nutrition, may form asexually many " heads," each of
Micivilomiimlm- wjiich, inside a future host, grows out into the long
nis. "™ "' series of joints which compose the lapewonn. In their
profuse asexual multiplication these parasites are like
parasitic fungi, but unlike them in the retention of the sexual process
as well.
In their asexual reproduction, the Polyzoa recall sponges, for not only
-do they all multiply by budding, and that aliundantly, but they form
peculiar winter-buds like sponge-gem mules, by which, on the death of the
parent, the continuity of life is nevertheless sustained. The winter buds or
Etatohlasts may further resemble sponge- gem mules in elaboialeness of ex-
ternal equipment, a common characteristic of passive resting structures.
In the higher bristle -fooled worm-types (Cbitopoda), asexual muUiplica-
ASEXUAL REPRODUCTION. ZOg
lion OMUTS in great vaiiely of expression. Some, when alarmed, brcal; up
inlo pieces, and olhers are known lo dg [his in npparenlly normal life.
Each pan in favourable conditions may reproduce ine whole. Thus, at
a comparatively high level among animali., repioduciion may l^ tilerally
rupture. Oflener, however, budding precedes the division, and cuiiims
cmiins of ringed worms are thus produced. Nor do the budded individuals
always keep in a straight line, but, as in the freshwater naids, may abut at
angles, and form a quaini living branch. Tu trhat degrees this irtrgulnrily
of budding may attain is well seen in the accompanying cut of a poitiun of
a worm {SyUii raaiesa). found on the "Challenger" voysRi. The buds
occnr lateially, terminally, or on any broken Euiiace, and the result is an
almost bush like compound organism rivalling even the hydrotds in the
freednm of its branching. Some of ihe branches become males or females,
and go leparale, or are sent adrift. In other syllids the separation of a
series of joints as a sexual individual has been repeatedly observed, or this
may be reduced till only one joiul, laden with reproductive elements, is set
free. In many of the Ch^slopods Ihe budding begins «hen the normal
siie of the individual has been slopped by unfavourable condilions, which
bring about separation, and the subsequent seiualily of ihe liberated indi-
viduals. When the entire individual is modified at the sexual period the
lerm tpigamy is used; this is illnstraled among Nereidx, Syllidese, and
Hesioni%. When only a part is modified sexually and is discharged as a
mouthleu free-swimming form, the lerm schizogamy is used. This is best
known in various species ai Syllit. In some cases, however, Exegoiu gim-
mifera and Au/ofylut hngt/tritm, both epi^amy and schiit^my are iltus-
Iraled in one life-eycle. (See A. Malaquin, " Epigamie et Schiii^amie
chez les Ann^lides," Zooi. Anutg., xix. (1896), pp. 420-423.) The facis in
regard lo reproductive processes in these Polyrhaet worms are indeed very
complex; we cannot do more than note three representative modes.
(o) In Ntrtis, for instance, certain of the s^menls become sexually
mature (epilokous) and differ markedly from Ihe nun -reproductive (alokous)
segments; but reproduction (usually fatal) occurs without any separation of
the epitokoui and alokous parts.
THE EVOUtTION OP SEX.
cliXlli^tr''" K^'"on
ASEXUAL REPRODUCTION. 311
time "like vermicelli soup "are headless. The inlact worm seems to live
in the crevices of the coral reef.
{t) In Syllidex theepilolcoussegmenlssepaisleand bud out ahead; the
alokous segments also survive, and have the povrcT of forming new s^menls.
SlacRshes and the like surrender their "arms" so readily, that some
have supposed that they might, in this way, normally muUipty. A volun-
tary surrender of parts as a mode of mulliplictlion is, hoivever, in this case
difficult to prove. It is said to occur in Cucuntariaplami, one of the sea-
cucumbers. While cruBlaceani, insects, spiders, and molluscs may lose and
T^row certain parts, no asexual multiplication occurs.
M of a leaf gf BrtBfkfJlMKi
In the tuoicates the asexual process has again full play. It is not eon-
lined to the passive sessile forms, where one might expect it, but occurs in
some of the free- swimmers us well. From a creeping stem buds may arise,
like plants from a rhizome; or a parent form may bud off all round, and
Rnally die away, leaving Ihe offspring in a drcle round a cavity. Both by
budding and division chains may be formed, as in the salpas. In these
lowly vertebrates asexual mulliplicaiion terminates. How the process
often alternates in rq^lar rhythm with oidinaiy sexual reproduction will
be discussed in the next chapter.
212 THE EVOLUTION OF SEX.
SUMMARY,
1. Artificial division may be readily utilised as a means of multiplica-
tion in plants and in lower animals.
2. Regeneration of lost parts is very common both in plants and
animals.
3. Asexual reproduction from continuous budding to discontinuous
multiplication has many degrees.
4. It occurs in many types from Protozoa to Tunicata.
LITERATURE.
General Works cited : the ordinal^ Zoolc^ical and Botanical Text-books.
Caullbry, M., and Mbsnil, F. — Les formes ^pitoques et revolution des
Cirratuliens. Ann. Univ. Lyon., XXXIX., 1890, pp. 1-200, 6 pis.
Fr£d£ricq. — La Lutte pour I'Existence chez les Animaux Marins. Paris,
1889. (For " Regeneration of Parts," &c.)
Haeckel.— Generelle Mornhologie. Berlin, 1866.
Lang, A. — Uel)er den Einftuss des Festsitzen auf den Thicren, und ueber
den Ursprung der ungeschlechtlichcn Fortpflanzung. Jena, 1888.
Spencer.— Principles of Biology. London, 1866.
CHAPTER XV.
Alternation ok Generations.
§ I. History of Discovery. — Early in the century the poet
Chamisso, accompanying Kotz-ebue on his circumnavigation of
the globe, observed in one of the locomotor tunicates {Salpd)
that a solitary form gave birth to embryos of a different char-
acter, connected together in chains, and that each link of the
chain again produced a solitary form. .Chamisso^s observation
does not seem to have been quite accurate, but there is no
doubt that he first called attention to what is by no means an
uncommon fact, that an organism produces an offspring very
unlike itself, which by and by gives origin to a form like the
parent. The progress of marine zoology and the study of
parasitic worms gave naturalists like Sars, Dalyell, Loven, Von
Siebold, and Ixuckart, early glimpses of many alternations in
life-history, but Steenstrup was the first to generalise the results.
This he did (1842) some twenty years after Chamisso, in a
work entitled " On the Alternation of Generations ; or. The
Propagation and Development of Animals through Alternate
Generations, a peculiar form of fostering the young in the
lower classes of animals." From hydroids and flukes, he gave
illustrations of the " natural phenomena of an animal producing
an offspring, which at no time resembles its parent, but which
itself brings forth a progeny that returns in its form and nature
to the parent." The interpolated generation he distinguished
by the name of "Amme" or "wet-nurse." In 1849, Owen
submitted Steenstrup's essay to stern criticism, rejecting especi-
ally the metaphorical name " nurse " as but a verbal explanation,
and proposing to explain what he also called "alternation
of generations," along with parthenogenesis and other pheno-
mena, by the supposition of a residual germ force or spermatic
power in the cells of the apparently asexual offspring. In
this he partially prophesied the modern conception of a
residual persistent germ-plasma. Soon afterwards Leuckart
214 1'HE EVOLUTION OF SEX.
attempted to treat all as cases of metamorphosis, thereby greatly
extending the meaning of that term. The labours of some of
the foremost naturalists have both extended Steenstrup's obser-
vations and rendered them more i)recise. We now know that
the phenomenon is of wider occurrence than was at first sup-
posed, and also that the title has been unduly extended to cover
dSJ
liiagraminatlc represenlation of alteniation of
generations, as, asexual generation ; jr,
sexual generation.
II. Shows alternation of asexual (as)
and sexual (s) generations.
In I. the sexual is becoming incrcas-
iiigly subordinated to the asexual (as iu
(lowerine plants).
In 111. the asexual is increasingly
subordinated to the sexual (in mosses).
several entirely different sets of facts. It is necessary, therefore,
to notice the various forms which the rhythm of reproduction
may take.
§ 2 . T/ic Rhythm between Sexual and Asex ual Reproduction. —
The clearest case to start with is that of many hydroids.
A sessile, plant-like zoophyte, which buds off numerous nutritive
persons, produces in the warm months modified individuals
which are set adrift as mcdusoid persons. Unlike the hydroid
which bore them, these become sexual; and from their fertilised
■■■ 215
ova an embrj-o develops, which eventually settles down to start
a. new sessile colony. And thus through the seasons we have
hydroid asexually producing sexual medusoids, and these again
producing hydroids. The life-history for two complete rhythms
may Ije wTitten in the formula, in which M, K, and A stand for
male, female, and asexual forms respectively, —
Or take, in slight contrast, the life-story of the common jelly-
fish Aurelia. I,arge free-swimming sexual animals produce ova
which are fertilised by s(ierms ; the embryo develops, not how-
Thfalmnnllnnnf |[«nrn>lli>nsinlhecoininnn itny-listi Aunfhi : i. the rmiwlinining fmhrva,
or planula ; j, ilm ciriliryo Bcttled doun ; i. i. j, 6, che dcnlnpini; iseiual <tagc, or h>-dn-
luba : J, I, ihe fomuimn of a pi1< ol individiuls : 9. ihc libcmion or thcM ; 10, 1 1, iba
acquuilinD ottba fna-livjng Miual niediua rorm.— ['nun HbcIlcL
2l6 THE EVOLUTlO!^ Ot SE^.
ever into a jelly-fish, but into a sessile hydroid-like organism or
" hydra-tuba." By growth and division this asexually produces
the jelly-fish in turn. Here the sexual generation is more stable
and conspicuous, the reverse of the former case, but the same
formula applies.
Or take a case from another class of animals, the marine
worms. Some of the syllids have the following life-history. A
worm remains asexual, never attaining either the external
characteristics or the internal organs of the sexual individuals.
It gives rise to these, however, by an asexual process of chain-
making. Sexual individuals are budded off from the asexual,
into which their fertilised ova in turn develop. This must, of
course, be distinguished from cases where asexual multiplica-
tion is only a phase preceding the acquisition of sexuality
The above cases are again expressible in the simplest formula.
Now . take a more complex case, from among the tuni-
cates, the highest point at which the genuine alternation can be
said to occur. From the single ovum of a sexual Salpa an
asexual individual develops which remains for a time connected
with its parent. It eventually escapes and becomes free-swim-
ming, and after a time forms a process or stolon on the surface
of which buds are formed. The buds develop and form a
chain of individuals. Then the stolon with its chain is set
free from the asexual parent, and the links of the chain become
sexual Salpse.
In the allied DoUolum the life-history is more complicated.
A sexual form gives rise sexually to tailed asexual larvae; the
larva loses its tail and forms two processes or dorsal and ven-
tral stolons; buds from the ventral stolon migrate to the dorsal
stolon and develop there into a second set of asexual forms;
the dorsal stolon also forms buds on its own account; some of
these individuals on the dorsal stolon are set adrift, and each
develops a ventral stolon from which the young sexual forms
arise. There are thus several asexual generations between one
sexual stage and the next.
§ 3. Alternation between Sexual and Degenera'e Sexual Reproduction. —
The cases we have just noticed are both easier to state and easier to explain
than others which are sometimes also included under the vague title of
** alternation of generations." The above alternations were between sexual
and asexual reproduction, and must be distinguished, vague as the boundary
must be, from alternation between the ordinary sexual process and a
degenerate form of the same.
The history of some of the flukes (Trematoda) may be taken as a first
ALTERNATION OF GENERATIONS. 21 7
illustration. The common liver-fluke (Disfomum or Fasciola hepaiica)
which causes the disastrous '* rot" in sheep has a life of vicissitudes. The
fertilised ovum gives rise to an embryo ; this passes from the sheep, which
its sexual parent infested, to the water by the field side. There it leads for
a few hours an active life, knocking against many things, but finally
attaching itself to a minute water-snail {Limtiaus truncafuius). Into this
it bores, losing its covering of active cilia with change of habit, and be-
coming much altered into a passive vegetative form known as a sporocyst.
Now this sporocyst sometimes divides; and if this were all, and the results
grew up into liver-flukes, we should have the old formula and less loss of
sheep. But direct development never occurs, and we may leave the casual
division at present out of account. Certain cells within the sporocyst
develop like parthenogenetic ova. They produce within the body of the
sporocyst another brood of what are called Redia, There may be several
generations of redise, but the final result is a brood of minute tailed organisms
\Cercaria\ which leave the water-snails, leave the water even, creep up
grass stems, and encyst themselves. There most wait for death, a few
for the attainment of adult sexual life if they chance to be eaten by a
sheep.
This cannot be accurately ranked as parallel to what occurs among the
above-mentioned tunicates, for the rediae arise from precocious reproductive
cells. These cannot be called ova, and there is no fertilisation, but yet the
process is not one of division, or of budding. It is a degenerate process of
parthenogenetic reproduction in early life. The facts may be again summed
up in a formula, which does not take account of the occasional division
of the "sporocyst."
A, asexual larvae ; S. sexual fluke ; the upper circles represent
the special reproductive cells ; fertilised ova at the base.
This alternation between sexual reproduction with the usual fertilisation,
and reproduction by means of special cells which yet require no fertilisation,
prevails in many plants, e.g.y ferns and mosses. From a fertilised egg-cell
the ordinary fern-plant, with which every one is familiar, develops. But
this is quite asexual, if we mean by that that it is neither male nor female,
and that it produces neither male nor female elements At the same time
it produces special reproductive cells, — not egg-cells exactly, any more than
those within the sporocyst were, but yet able to develop of themselves into
a new organism. This is not another fern-plant, however, but an incon-
spicuous green organism, much less vegetative, and sexual. The so-called
"spore " formed on the leaves of the sexless fern-plant falls to the ground,
develops a ** prothallus," which bears male or female organs, or both. An
egg-cell is fertilised by a male element, and the conspicuous vegetative fern-
plant once more arises. The formula is therefore as follows: —
2l8
THE EVOLUTION OF SEX.
Where A = sexless vegetative fcrn-plant ;
sp. sthe parthenogeneiic special reproductive cell or spore;
Ssthe sexual inconspicuous " prothallus," with male and female organs.
Now take the history of a moss. .Unlike the fern, the more conspicuous
" moss-plant " is sexual. It bears male and female organs, and an egg-cell
is fertilised by a male element. The fertilised egg-cell, however, does not
lose its hold of the mother plant, but grows like an encumbering parasite
.u[)on it. Obviously, then, it does not give rise to another ** moss- plant."
The result of the fertilised egg-cell is a tiny sexless stalk, which bears on its
apex the special reproductive cells or spores with which we are now familiar.
In other words, the fertilised egg-cell develops into a parasitic spore- bearing
generation. The •** spores '* fall into the ground, as they did in the fern,
and there grow into a usually thread-like structure, from which the sexual
moss-plants are budded off. If we do not emphasise the transitional thread-
like stage, — the protonema as it is called, — the formula is as follows: —
Where A = inconspicuous sexless parasitic generation upon the "moss-plant " ;
bp. = the special p;irthenogeiietic reproductive cell or spi^re produced by A ;
S = the conspicuous sexual " moss-plant," budded from the threads developed
from the spore.
If we do emphasise the ** protonema " stage {/), and regard the moss-
plants as ascxually budded from it, the formula runs: —
In the fern, the vegetative sexless generation was ihe more conspicuous;
in mosses, the sexual generation. In a way this recalls the contrast
l)etween the life-history of many a zoophyte, and that of the common
jelly-fish Atirelicu The asexual hydroid colony is more conspicuous than
ALTERNATION OF GENERATIONS.
219
the usually small swimming-bell, but the sexual jelly-fish is much more
conspicuous than the minute asexual "hydra- tuba." The common com-
parison between medusoid and hydroid on the one hand, and prothallus
and fern-plant on the other, is rather misleading, simply because the
hydroid merely buds off the medusoid, while the fern-plant produces the
prothallus from a special reproductive cell or spore. In some ferns and
mosses, however, a more exact parallel is occasionally exhibited. The
production of "spores" may be suppressed, and from the place where they
should be formed a (sexual) fern-prothallus or a new (sexual) moss-plant is
vegetatively developed, just as medusoid from hydroid. This exceptional
occurrence is technically called apospory. The very opposite of this also
occurs, the suppression not of the spore- bearing, but of the sexual genera-
tions. The fern-plant then arises vegetatively from the prothallus; and
this would be paralleled if we supposed the sporocyst of the fluke to bud
off rediae (as it sometimes tlocs), and these to continue the species without
ever becoming really sexual, solely by means of the special cells above
described. This is called apogamy.
Apogamy has also been described by Treub in the remarkable flowering
plant Balaftof>hora^ where the abortion and disappearance of the egg-
apparatus is closely parallel to what occurs in one of the brackens {Pteris
cretica), ("Ann. jard. Bot. Buitenzorg," xv., 1898, pp. 1-25, 8 plates.)
§ 4. Combination of both these Alternations. — The asexual hydroid buds
off a medusoid, the fertilised ovum of which develops into a hydroid.
Here there is simple alternation between sexual and asexual reproduction.
A sexless fern-plant forms special reproductive cells (spores), which
develop parlhenogeneticaliy into a sexual prothallus, from the ferliliicd
egg-cell of which the fern-plant arises.
The difference between these two alternations has been as often pointed
out as it has been ignored. The former is called true alternation of
generations (or metagenesis); the latter is called by zoologists, in reference
to flukes for instance, heterogamy. Comparisons between the alternations
in plants and animals have seldom recognised the distinction*
220 THE EVOLUTION OF SEX.
I^t it be recognised, however, and we can readily proceed to more
complicated cases where the two are combined. Returning to the liver-
fluke and others like it, we find that the sporocyst sometimes multiplies in
a genuinely asexual fashion — without the intervention of precocious ova,
special reproductive cells, germs, or spores, as they may be called —
by direct division or budding. For such cases the formula must be
modified as follows: —
The complication is not serious. It is simply that, before the multipli-
cation by special cells sets in, there may be more than one (A', A") entirely
asexual (and not merely sexless) generation.
§ 5. Alternation of JuveniU Pttrthenogenetic Reproduction with the
Adult Sexual Process. — We have already noted the curious precocity of
some midge larvse, which reproduce while still young. Cells within the
body, apparently precocious ova, develop parthenogenetically into larvae,
which prey upon the mother larva, eventually kill her and leave her, only
themselves to become in turn similar victims of precocity. This may con-
tinue for a series of generations, with continuous decrease in the size of
the reproductive cells, till finally true sexuality and adult life is attained.
The reproductive cells here are rather more differentiated than those in the
young flukes, but the close parallelism is indubitable. Except that there is
for a while no fertilisation, the process can hardly be called asexual. The
formula may be expressed in a gentle curve : —
Where the starting-point is as before a fertilised ovum ;
L= prematurely reproductive larva;
ps = precocious par thenogenetic ' ' pseudova " ;
Ssadult sexual male or female organism.
Soniewhat different is the curious case of Gyrodactyhis^ a trematode
parasitic on fresh- water fishes, where three generations are found enclosed,
one within the other, in a fashion which recalls the fancies of the preforma-
tionists. In this case, however, it seems likely that internal fertilisation
really occurs.
§ 6. Alternation of Partlunogenesis and Ordinary Sexual Reproduction.
— In our gradual ascent, we now reach the frequent allernation of partheno-
genesis and ordinary sexual reproduction. The special cells which develop
without fertilisation are now genuine parthenogenelic ova, and the organ-
isms which produce them are adults, not juveniles. The formulae will
ALTERNATION OF GENERATIONS. 221
differ mainly in the number of generations through which the partheno-
genesis may be continued.
Where the starting-point is a fertilised ovum ;
P=parthenogenetic female^ producing a parthenogenetic
ovum, from which anse other parthenogenetic forms,
or eventually ;
S a male and female.
§ 7. Alternation of Different Sexual Generations, — The rhythm may
be followed in yet a higher scale. In a very few cases there is an alter-
nation between two different sexual generations. Thus one of the thread-
worms {Lepiodera appendiculaicC) found in the snail gives rise, by the
ordinary sexual process, to a different form, which leads a free life, and
subsequently gives origin to the parasite. In both generations the sexes
are distinct. More remarkable still is the history of another nematode
{Angiostomum nij^rovenosum), found in the lung of the frog. It is physio-
logically hermaphrodite, though its organ is ovary-like; its eggs are
fertilised by its own sperms, which mature Hrst; the progeny become sexual
— males and females — in the earth, and their offspring return to the frog,
where they become hermaphrodites. Another example of alternation of
sexual generations is found in one of the threadworms which occur in man
{Rhabdonema strongy hides),
§ 8. Occurrence of these Alternations in Animals, — From sponges to
tunicales such alternations occur. Beyond the latter, unless we wish to be
very subtle, they cease. It is necessary to be clear about the fact that
asexual and sexual reproduction may occur together in the same form.
The common hydra gives off buds in an entirely asexual way, but it is also
a sexual animal, with male and female organs. There may be periods of
vegetative growth and climacterics of sexuality in the same organism, with-
out any alternation of generations
It is possiltle that the term alternation of generations may be applied
to some of the phenomena observed in the Protozoa. Thus Brandt main-
tained that all the colonial radiolarians, known as Sphserozoa, form on the
one hand isospores^ which are all equal and apparently parthenogenetic,
and on the other hand anisospores, which are large and small, — in fact,
sexually dimorphic He believes — though the fact cannot be called
demonstrated — that two unequal anisospores unite to form a double cell, a
fertilised unit, which will produce isospores again, and these the normal
colony. The generation of these Sphserozoa is further complicated {a) by
division of the colonies, (b) by division of the individuals of young vegetative
222 THE EVOLUTION OF SEX.
colonies, and {c) by the formation of special ** extra-capsiilar " reproductive
bodies in young colonies.
The history of the common fresh-water sponge {Sf>ongilla), as told by
Marshall, is one of many vicissitudes. In autumn the sponge begins to
suffer from the cold and scarcity of food. It dies away ; but some of the
units save themselves, and, in a sense, the parent, by forming the
** gemmules " we have already noticed. These winter in a quiescent slate
within the parental corpse, but in spring they get out of the debris, and
start male or female sponges. The males are short-lived, but their male
elements fertilise the ova of the females. The fertilised ovum develops
into a ciliated embryo, and this into an asexual sponge, which produces the
gemmules.
The starting-point a fertilised ovum, which develops into
A =3 asexual sponee, which forms only
Gsgemmules, which develop into
Ssnxale and female sponges.
Besides the hydroid and medusoid, the hydra-tnba and jelly-fish alterna-
tions, which we have already noticed, there are many complications of
degree among coelenterates. The medusoid stage degenerates by subtle
gradations, ceasing to be free, and eventually becoming what, if its history
were not known, would be called an organ rather than a ** person " of the
colony. Furthermore, it may itself take to budding, and continue the
asexual habit of the hydroid from which it springs. Outside the Hydrozoa,
genuine alternation of generations does not occur, unless that described by
Semper for Fungia corals be accepted as such.
A very interesting alternation has been described by W. K. Brooks in
a remarkable medusa (Epenthesis macrcuiyi). On the reproductive organs
of this swim -bell there grow, like parasites, what are exactly comparable
to the reproductive buds (blastostyles) of a hydroid, and these form medu-
soids by budding. The result is a compound colony, which approaches the
Siphonophora. The process recal Is and surpasses the apogamy of a fe w ferns.
Among worm-types, the strict alternation of generations in some of the
marine chsetopods (syllids), the more complicated phenomena of so many
trematodcs, the sexual rhythms of that peculiar threadworm Angiostomum,
have been already discussed. It is necessary, however, to state the case for
tapeworms, which are usually included among the examples of alternation
of generations. The usual view is, that the embryo of a tapeworm develops
into an asexual bladder-worm, which asexually buds off a ** head," or more
than one. Such a "head," passing to another host, buds off asexually
the chain of reproductive joints or sexual individuals which constitute a
tapeworm. Asexual bladder-worm, asexual **head," and sexual joints,
form the series. That there is a genuine alternation of generation is
believed by somje authorities, but there are emphatic difficulties against
this supposition, except in the occasional occurrence of a bladder-worm
with several ** heads," each of which may develop into a tapeworm. The
ALTERNATION OF GENERATIONS. 223
case is well stated by Hatchett Jackson in his monumental edition of
Rolleston's " Forms of Animal Life," and we accept his verdict that there
is really one individual throughout, except when asexual multiplication of
heads occurs. The tapeworm, on this view, is an adult sexual bladder-
worm, and the joints are only highly individualised segments.
Of the parthenogenetic cycles in crustaceans and insects, the juvenile
reproduction of some of the latter, and the true alternation of generations
in some tunicatcs, enough has already been said.
Von Jhering is responsible for starting the paradox that in higher
animals a mother may bring forth her grandchildren. He refers to the
case of the hyaena-like edentate rraoptiSy where a single ovum gives rise to
eight embryos, which are thus in a pedantic sense grandchildren I The
frequent occurrence of twins in all groups, the remarkable case of an earth-
worm {Lumbr/cus trapezoides) in which a double embryo is constant, and
the morphological resemblance of polar globules to abortive germs, led Von
Jhering to maintain that the origin of multiple embryos from a single
ovum is the primitive and normal condition, and that the development of
only one is secondary and adaptive. The data are hardly sufficient for such
a striking conclusion.
Paul Marchal has described in regard to Encyrtus fuscicoUis^ one of the
parasitic Ilymenoptera, 'which lays its eggs in those of another insect,
Hyponomeutes^ the remarkable peculiarity that the ovum gives rise not to
one embryo, but to *'a legion of small morulx," forming a chain of 50-100
embryos. The amniotic envelope loses its vesicular form and becomes a
long flexible tube, within which lie the embryos surrounded by granular
material apparently derived from a disruption of part of the original egg.
As the author says, this is a mode of reproduction unique among Arthro-
pods, if not among animals. {** Comptes Rendus," cxxvi., 1898, pp. 662-4. )
§ 9. Beard's Hypothesis of Aiternaiion in Vertebrates, — Beard has
propounded a somewhat subtle theory which suggests that there is a
disguised alternation of generations in vertebrate animals, just as there
is in flowering plants. In several invertebrate types, e.g, Echinodcrms
and Phoronis, the egg gives rise to a larva which does not directly develop
into the definitive organism, but serves as the foundation on which the
development recommences as it were de novo. Similarly, Beard believes
that in Vertebrates, whether skate or chick, there are traces of an asexual
larval stage on the top of which the embryo proper develops. At the
*' critical stage," when the embryo begins to put on Us generic and specific
characters, it also sets about the task of suppressing the larval foundation.
("On Certain Problems of Vertebrate Embryology," Jena, 1896, vi. and
77 PP- )
§ 10. Occurrence of Aiternaiion s in Plants, — In the lower
plants, algae and fungi, an alternation between spore-producing
and truly sexual generations is frequent In mosses and ferns it
is almost constant, and yet more marked. Occasionally either
spore-formation or sex-cell formation may be suppressed, and
the life-history thus simplified. In a few of the higher plants
both are exceptionally suppressed, and we have thus a reversion
to a purely vegetative process, just as if a hydra went on giving
off daughter-buds without ever becoming sexual. In the
224 THE EVOLUTION OF SEX.
flowering plants, what corresponds to the sexual generation of
a fern is much reduced; it has come to remain continuous with
the vegetative asexual generation, on which it has reacted in
subtle physiological influence. Just as in the higher animals,
alternation of generations finds at most only a rudimentary
expression.
§ II. Heredity in Alternating Generations. — The problem
of the relative constancy of inheritance is now in part solved by
the theory of germinal continuity. The ovum which develops
into an offspring is virtually continuous with the ovum which
gave rise to the parent. A chain of ovum-like cells is only
demonstrable in a few cases; but Weismann overcomes this
difficulty, by supposing that what really keeps up the proto-
plasmic tradition or continuity between the parental ovum and
the next generation, is a specific and stable portion of the
nucleus, — the "germ plasm." When a medusoid goes off from
a hydroid, it carries with it a legacy of this germ-plasm, con-
tinuous with that which gave rise to the hydroid. This legacy
forms the reproductive elements of the medusoid, which in
turn give rise to hydroids. The medusoid itself is a modified
asexual growth, into which some of the germ-plasm of the
hydroid has migrated ; it is literally only the bearer or trustee of
the hydroid germ-plasm. Weismann's classic researches on
hydroids have shown that the reproductive cells, which, by
hypothesis, bear the germ-plasm, often arise far down in the
hydroid body, and actually migrate to their final seat in the
bearer. Where the alternation is not between sexual and
asexual, but between the ordinary sexual process and multipli-
cation by special parthenogenetic cells, as is the case in many
flukes, we may in the same way suppose that the cells within a
sporocyst which give rise to rediae are, like ova, charged with
this reproductive germ -plasm. It is very interesting to notice
that, as far back as 1849, Owen had a distinct prevision, not
only of the distinction between body-forming cells and repro-
ductive-cells, of which so much is now made, but of the essential
idea of the " germ-plasm." Speaking of the recurrence of a
parental form after numerous interpolated generations, he says,
"the essential condition is the retention of certain of the
progeny of the primary impregnated germ-cell, or, in other
words, of the germ -mass unchanged in the body of the first
individual developed from that germ-mass, with so much of
the spermatic force inherited by the retained germ -cells from
ALTERNATION OF GENERATIONS. 225
the parent cell or germ-vesicle as suffices to set on foot and
maintain the same series of formative actions as those which
constituted the individual containing them." In this some-
what over-weighted sentence, if we read " germ-plasm " instead
of ** spermatic force," we have a close approximation to the
modern conception of Weismann. Again, Owen says, "an
impregnated germ-cell imparts its spermatic power to its cell-
offspring; but when these perish, or when the power is
exhausted by a long descent, it must be renewed by fresh
impregnation. But nature is economical, and so long as
sufficient power is retained by the progeny of the primary
impregnated vesicle (the essential part of an ovum), individuals
are developed from that progeny without the recurrence of the
impregnating act"
§ 12. Hints as to the Rationale of Alternation, — The puzzle
of alternation may be lessened if we consider the physiological
aspect of the facts. A fixed hydroid contrasted with a swim-
ming-bell or medusoid, a sessile hydra-tuba contrasted with an
actively locomotor jelly-fish, illustrate not a peculiar antithesis,
but a most general and fundamental rhythm of organic life, —
that between nutrition and reproduction. The hydroid has a
relatively passive habit and a copious nutrition ; it is prepon-
deratingly vegetative and asexual. The reverse habit, the
physiological rebound, finds expression in the medusoid. In
the same way, though the alternation is less strictly between
asexual and sexual, the contrast between leafy spore-bearing
fern-plant and inconspicuous sexual prothallus is again funda-
mentally parallel. The notation adopted must have already
suggested our fundamental diagram, the different forms of which
may be separated out or superposed ; —
SUM OF FUNCTIONS.
Nutrition. Reproduction.
Anaboltsm. Kataboliun. Female. Male.
15
226 THE EVOLUTION OF SEX.
Although it has just been shown that the process of alter-
nation demands a much more thorough analysis and discrimi-
nation of the different cases than has hitherto been customary,
and this on the physiological as well as merely on the morpho-
logical side, the general aspect of the process, in which an
asexual form alternates with one or more dimorphic sexual
generations, makes it evident that we have here to do in two
generations with what is often so obvious in one, — the familiar
antithesis between nutrition and reproduction. A consideration
of the physiological distinctions between the asexual and sexual
generations, shows that the former is the expression of favour-
able nutritive conditions resulting in vegetative growth, or at
most in asexual multiplication, while the latter is conditioned
by less propitious circumstances. Just as a well-nourished
plant may continue propagating itself by shoots and runners,
and just as an aphis in artificial summer may for years repro-
duce parthenogenetically, so a hydroid with abundant food and
otherwise favourable environment may be retained for a pro-
longed period vegetative and asexual, while dearth of food and
otherwise altered conditions evoke the appearance of the sexual
generation. The contrast between the deeply-rooted well-
expanded fern-plant and the weakly-rooted slightly-exposed
prothallus, is obviously that between an organism in conditions
favourable to the continuance and preponderance of anabolic
processes, and an organism in an environment where katabolism
is, at an early stage, likely to gain relative ascendency. The
former is thus naturally asexual, the latter sexual. A survey,
in fact, of the conditions and characteristics of the two sets of
forms, inevitably leads us to regard the asexual generation as
the expression of relatively predominant anabolism, and the
sexual as equally emphatically katabolic. Alternation of genera-
tions in its less complex forms may thus be described as a
rhythm between a relatively anabolic and katabolic prepon-
derance.
§ 13. Orif>!n of Alternation of Ctnerations. — ^Even in an individual
plant or animal there are vegetative and reproductive periods ; alternation
of generations involves the separation of these to different individuals, by
the interpolation of more or less asexual reproduction. In most hydroids,
the asexual vegetative tendency preponderates; in most medusoids, great
activity is dominant. But the origin in each particular case is involved
in the pedigree of the organism. Thus Haeckel distinguishes a progressive
from a retrogressive origin; in the former, the organisms are in transition
from preponderant asexual to sexual reproduction; in the latter, the
ALTERNATION OF GENERATIONS. 227
organisms are returning or degenerating from dominant sexuality to an
asexual process. It is safe to say that the latter is more frequently the
right interpretation of the facts. So far as reproduction is concerned, one
of those medusoids (Trachymedusae) which have no corresponding hydroid
parent, or a jelly-fish like Pelagia which has no fixed asexual hydra-tuba
stage, is nearer the ancestral habit than those members of both divisions
which exhibit alternation of generations. In regard to alternating series of
similar forms with different d^rees of sexuality, e.g.^ parthenogenesis and
true sexual reproduction in aphides, Weismann suggested that the alterna-
tion may be associated with the periodic action of external influences
(" Studies in the Theory of Descent," chap. v.). But in contrast to such
cases he distinguished, (a) an origin from metamorphosis, where one stage in
the life-history becomes precociously reproductive, e.g,^ in the midge
Cecidoniyia; (b) the case of the Hydromedusse, where sexuality is post-
poned in early life, and asexual reproduction dominates ; and {c) an origin
from division of labour within a colony. Without entering upon a discus-
sion of each case in relation to its history and environment, it is not possible
to do more than reassert that in many different degrees the continuous alter-
nation between growth and multiplication, nutrition and reproduction,
asexuality and sexuality, anabolism and katabolism, may express itself in
the life-history of the organism.
228 THE EVOLUTION OF SEX.
SUMMARY.
1. The fact that successive generations may be markedly diflferent was
observed by the poet Chamisso, and first made precise by the zoologist
Steenstrup.
2. A fixed asexual bydroid buds off and liberates locomotor sexual
medusoids, whose fertilised ova give rise again to hydroids. Asexual and
sexual generations alternate.
3. The offspring of the liver-fluke forms from certain cells in its body
a numerous progeny; these repeat the same process several times; the last
generation grow into the sexual liver-flukes. Reproduction by special
cells, like precocious undifferentiated ova, alternates with reproduction by
ordinary fertilised ova. So too the vegetative sexless ** fern- plant" gives
rise to spores, which develop into an inconspicuous sexual prothallus.
From the fertilised egg-cell of^the latter the "fern-plant" arises.
4. These two different kinds of alternations (§ 2 and § 3) may be com-
bined in a more complicated manner.
5. In some flies precocious parthenogenetic reproduction alternates with
the normal sexual reproduction of the adults.
6. In many insects and crustaceans, parthenogenetic reproduction alter-
nates with the normal sexual process. There may be one or many inter-
vening parthenogenetic generations.
7. A hermaphrodite threadworm parasitic in the frog fertilises its own
eggs, which develop into free-living males and females, from the fertilised
ova of which the hermaphrodite parasites again arise. Here there is an
alternation of sexual generations.
8. The occurrence of these various alternations is widespread, from
sponges to tunicates.
9. Beard's hypothesis of alternation in vertebrates.
10. In plants they occur in algae and fungi, are almost constant in ferns
and mosses, but are inconspicuous in higher plants.
11. The problem of heredity is somewhat complicated by such alter-
nations.
12. Alternation of generations may be described as a rhythm between
a relatively anabolic and katabolic preponderance.
13. Alternation of generations has probably arisen in various quite
distinct ways.
LITERATURE.
See the general works already cited; also, Steenstrup ** On the Alternation
of Generations," transl. Ray Soc, 1845; Owen's *' Parthenogenesis,"
&€., 1849; Haeckel's *' Generelle Morphologie," 1866; Weismann,
A., Die Entstehung der Sexualzellen bei den Ilydromedusen, Jena,
1883, and Papers on Heredity (Translation), Oxford, 1889; Vines'
article '* Reproduction — VegetJible," Ency. Brit.
ALTERNATION OF GENERATIONS. 229
Beard, J. — On the Phenomena of Reproduction in Animals and Plants,
on Antithetic Alternation of Generations, and on the Conjugation of
the Infusoria. Anat. Anzeiger, XL, 1895, pp. 234-255, 5 6gs.
Bower, F. O. — On Apospory and Allied Phenomena. Trans. Linn. Soc
Botany, 1887.
Brooks, W. K.— Life- History of Epenthesis McCradyi n. sp. Stud. BioL
Lab. John Plopkins Univ., IV., 1888, pp. 147-62, 3 pis.
Chamisso, A. DE. — De Animaiibus quibusdam e classe vermium Lin-
nseana in circumnavigatione tense auspicanti comite N. RomanyofF
duce Ottone de Kotzebue annis 1815, 1816, 1817, 1818 peracta.
Fasc I. De Salpa, Beroline, 18 19.
BOOK IV.
*•►<•«
THEORY OF REPRODUCTION,
CHAPTER XVI.
Growth and Reproduction.
§ I. Facts of Growth, — In a well-known aphorism Linnaeus
noted that living organisms are not alone in their power of
growth. Crystals become centres for other crystals, till a large
mass results; and the product, as every case of minerals shows,
is often both orderly and complex. But there are great
differences between organic and inorganic growth, one of the
most obvious being that the organism is able to grow larger at
the expense of material different from itself, while the crystal
can only increase out of material similar to itself. The grass
grows at the expense of water, soil, and air; and the foal grows
at the expense of the grass, but a dead thing cannot grow in
this way. Probably of much less importance is the old dis-
tinction that the growth of organisms has a peculiar method of
its own, that of intussusception as distinguished from mere
accretion. The new particles which are taken in, more than
replacing previous expenditure, are not deposited upon the
surface of already established material, as is the case with a
crystal, but are intercalated in the interstices of previous
particles. We cannot enter here upon the long-continued
controversy, whether such structures as the cell-wall and starch-
grains of plants grow thicker or larger by accretion in crystal-
like fashion, or by intercalation which is supposed to be
characteristically organic. It is worth noticing, however, as
Biitschli points out, that if the living matter has the form of an
intricate network, the fresh material of replacement or growth
may be added to the surfaces of the threads which make the
web. Thus what is roughly called intercalation may be more
literally an internal accretion.
Hunger is a dominant characteristic of living matter. When
a unit mass or cell has been giving off energy in doing any kind
of work, its substance is chemically impaired, — less capable of
doing further work until new energy has been supplied by
234 THE EVOLUTION OF SEX.
nutrition. The supply which the lifelong hunger of the proto-
plasm demands, is frequently afforded in greater abundance
than the actual necessities require. There is a surplus for
further upbuilding after mere reparation has been made. This
surplus is the condition of growth. In other words, growth or
addition to the capital of the organism occurs when income is
in excess of expenditure, when construction preponderates
over disruption.
But beside this familiar fact, it is necessary to place another
certainty, that of the limit of growth. There are among the
Protozoa a few giants, such as the large amoeboid Pelomyxa^
some of the gregarines, and even more markedly the extinct
nummulites, which were sometimes as large as half-crowns. So
an occasional alga, like Botrydiutn^ may swell out into a large
single cell, and the ova of animals, e.g, birds, are often greatly
expanded by the accumulation of yolk. Yet cells generally
remain very small. They have their maximum size, approxi-
mately constant for each species. Up to this point they grow,
but no further. The same, as every one knows, is true of most
multicellular animals. The size fluctuates slightly according to
the conditions of individual life, but the average is strikingly
constant
§ 2. Spencer's Theory of Growth. — The first adequate dis-
cussion of growth is due to Spencer. He pointed out, that in
the growth of similarly shaped bodies the increase of volume
continually tends to outrun that of the surface. The mass of
living matter must grow more rapidly than the surface through
which it is kept alive. In spherical and all other regular units
the mass increases as the cube of the diameter, the surface only
as the square. Thus the cell, as it grows, must get into physio-
logical difficulties, for the nutritive necessities of the increasing
mass are ever less adequately supplied by the less rapidly
increasing absorbent surface. The early excess of repair over
waste secures the growth of the cell. Then a nemesis of grow-
ing wealth begins. The increase of surface is necessarily
disproportionate to that of contents, and so there is less
opportunity for nutrition, respiration, and excretion. Waste
thus gains upon, overtakes, balances, and threatens to exceed
repair. Suppose a cell to have become as big as it can well be,
a number of alternatives are possible. Growth may cease, and
a balance be struck ; or the form of the unit may be altered,
and surface gained by flattening out, or very frequently by
GROWTH AND REPRODUCTION. 2^$
outflowing processes. On the other hand, waste may continue
on the increase, and briny about dissolution or death ; while
closely akin to this, there is the most frequent alternative, that
the cell divide, halve its mass, gain new surrace, and restore the
balance. Independent suggestions, similar to Spencer's, have
been made by Professor Leuckart and Dr. Alexander James.
§ 3. Cell-Division. — What usually occurs, then, at the maxi-
mum or limit of growth, is that the cell divides. This, in its
simplest forms, is rough enough to suggest rupture or overflow;
but in the vast majority of cases it is an orderly and definite
process, in which the nucleus plays an important part, and the
centrosomes behave as if they were intimately concerned with
the mechanism of division. By a complicated series of changes,
both in form and position, the essential nuclear elements group
themselves so as to form the daughter-nuclei of each product
of division. That attractions and repulsions do exist within
the cells is certain; an analysis of their precise nature— the
final problem of histology— is still far in the distance. We
2^6
THE EVOLUTION OF SEX.
cannot get within miles of it. The problem has always loomed
before embryologists and histologists, — the historians and
mechanicians of the organism. Pander, in the first quarter of
this century, was inquiring into the mechanics of development,
Illustrating the Mechanism of CcII-Division.-HC^) the
chromatin elements of the nucleus forming an
"equatorial plate" la the lower figure, drawn
towards the poles to form two daughter-nuclei in
the upper ; (o) part of the intricate system of fine
plasmic threads ; (c) the centrosomes from which
these radiate.— From Boveri.
and Lotze followed him with some luminous suggestions. The
task has been continued by numerous modern workers;
various " laws of cell-division " have been formulated ; many
attempts to elucidate the '* mechanism" have been made;
GROWTH AND REPRODUCTION. 237
but the inquiry is still tentative. The reader may be referred
to treatises on the cell by Wilson, Hertwig, and others.
§ 4. Protoplasmic Restatement — In the above helpful sug-
gestion, Spencer has emphasised the reasonableness and general
necessity of cell-division at the limit of growth, refraining from
the deeper question of the actual mechanism involved. In
truth such cautious reserve must still be maintained, but
Spencer's analysis perhaps admits of being expressed in lower
terms. The early growth of the cell, the increasing bulk of
contained protoplasm, the accumulation of nutritive material,
correspond to a predominance of protoplasmic processes
which are constructive or anabolic. The growing disproportion
between mass and surface must however imply a relative
decrease of anabolism. Yet the life, or general metabolism,
continues, and this entails a gradually increasing preponder-
ance of destructive processes, or katabolism. As long as
growth continues, the algebraic sum of the protoplasmic pro-
cesses must of course be plus on the side of anabolism, and
growth may be now more precisely defined as the outcome of
the preponderance of an anabolic tendency, rhythm, or bias.
The limit of growth, when waste has overtaken and is begin-
ning to exceed the income or repair, corresponds in the same
way to the maximum of katabolic preponderance consistent
with life. The limit of growth is the end of the race between
anabolism and katabolism, the latter being the winner. Thus
cell-division in plants occurs especially at night, when nutrition
is at a standstill^ and when there is therefore a relative kata-
bolic preponderance.
What is true for the cell, is true for cell-aggregates.
Organisms in their entirety have very definite limits of growth.
Increase beyond that takes place at a risk, hence giant varia-
tions are peculiarly unstable and short-lived. Or again, just as
the single cell has found, probably somewhat pathologically, a
surface-gaining expedient in the emission of mobile processes,
so many organs, notably leaves, have struck a balance between
mass and surface by becoming split up into lobes and more or
less discontinuous expansions.
Spencer has laid great stress on the importance of the
physiological capital with which the organism begins; this
represents, in active animals at least, the start which their
anabolism gets at the outset. Other things equal, growth
varies — (a) directly as nutrition ; (b) directly as the surplus of
238 THE EVOLUTION OF SEX.
nutrition over expenditure ; (^r) directly as the rate at which
this surplus increases or decreases ; (c/) directly (in organisms
of large expenditure) as the initial bulk ; and {(s) directly as
the degree of organisation, — the whole series of variables
being finally in close relation to the doctrines of the persist-
ence of matter and conservation of energy. Some apparent
exceptions are readily explained. Thus, many plants seem to
grow indefinitely, but they expend very little energy, and have
often enormous surface area in proportion to mass. The
crocodile goes on slowly growing, though at a gradually di-
minishing rate, but it again expends relatively little energy in
proportion to its high nutrition. In many fishes, however, of
great activity, the limit of growth seems to be very indefinite.
Birds which expend most energy have their size most sharply
defined.
§ 5. TAe Antithesis between Grmvth and Multiplication^
between Nutrition and Reproduction. — The life of organisms is
conspicuously rhythmic. Plants have their long period of
vegetative growth, and then suddenly burst into flower. Ani-
mals in their young stages grow rapidly, and as the growth
ceases reproduction normally begins. Or again, just as peren-
nial plants are strictly vegetative throughout a great pieut of
the year, but have their stated recurrence of flowers and fruit,
so many animals for prolonged periods are virtually asexual,
but exhibit periodic returns of a reproductive or sexual tide.
Foliage and fruiting, periods of nutrition and crises of repro-
duction, hunger and love, must be interpreted as life-tides,
which will be seen to be but special expressions of the
fundamental organic rhythm between sleep and waking, rest
and work, upbuilding and expenditure, which are expressed on
the protoplasmic plane as anabolism and katabolism.
The common hydra, in abundant nutritive conditions, pro-
duces numerous buds, and even these sometimes begin them-
selves to bear another generation. In other words, we may
almost say, with plenty of food the polype grows abundantly,
so obviously is this asexual reproduction continuous with
growth. A check to the nutritive conditions, however, brings
on the development of the sexual organs and the occurrence
of sexual reproduction. In planarian worms, the asexual
multiplication of which we have already noted, Zacharias ob-
served that favourable nutritive conditions were associated
with the formation of asexual chains, while a check to the
GROWTH AND REPRODUCTION. 239
nutrition brought about both the separation and the sexual
maturity of the links. Rywosch corroborates this, noting in
Microstomum lineare that the generative organs do not become
completely matured till the individuals cease to be links in a
chain, and that the sexuality is hastened by outside influences
such as checked nutrition. The gardener root-prunes his
apple-tree, thereby checking nutrition to improve the yield of
fruit, in other words, to augment reproduction. Reversely, the
removal of reproductive organs may increase the development
of the general " body " both in plant and animal, — witness
the castrated ox, capon, &c., or the way in which the gardener
nips off the flower-buds from his foliage plants. Taking a
further step, we recall the familiar and already repeated fact,
that favourable nutritive and other conditions enable the
aphides to continue parthenogenetic through the summer
months ; but both for the common plant-lice and for the vine-
insect phylloxera, it has been shown that a check to nutrition
causes the parthenogenesis to cease, and is associated with the
return of sexual reproduction. The above instances are ob-
viously not all upon the same plane. They illustrate however,
at different levels, the same great contrast. It is necessary,
however, to become more precise.
§ 6. The Contrast between Growth and Reproduction in the
Individual, ^(ai) The Distribution of Organs, — ^The general
position of the flower at the end of the vegetative axis is so
obvious a fact that its import tends to be overlooked. The
end of the axis is furthest from the source of nutritive supply;
with exaggeration, we might call it the starvation-point.
There, with katabolic conditions tending relatively to pre-
dominate, the reproductive organs are situated. The flower
occupies a katabolic position, and is often the plant's dying
effort.
In the tiger-lily, growth at first tends to remain continuous,
and the base of the bulb bears simple vegetative buds
Further up, however, where nutrition reaches its maximum
the axils of the leaves contain buds, which are separable
though still asexual. Finally, further up still, where nutrition
is relatively less active and katabolism is maximised, the for-
mation of flowers indicates the appearance of sexual repro-
duction.
In many ferns, the contrast between the vegetative and re-
productive regions of the organism is as marked as in the
240 THE EVOLUTION OF SEX.
flowering-plant Thus the moonwort (Botrychiunt) and the
adder's tongue (Ophiogiossum) have their spore-bearing shoots
standing in conspicuous antithesis to the leafy portion, and a
similar contrast is well seen in the royal fem {Osmuncfa) and
some of its allies.
In animals, the contrast in position between reproductive
organs and the general body is never so marked. Yet the
The Moonwoil Fern (^B/rfe*-'""
ini?^ 'frond (i), >nd frucii
generally posterior position of the organs, their frequent close
association with the excretory system, their occasional rupture
as external sacs, must not be lost sight of.
(^) 2/ie Contrmt in l?ie Individual //>.— Growth during
youth, sexual maturity at the limit of growth, the continued
alternation of vegetative and reproductive periods, are common-
GROWTH AND REPRODUCTION, 24 1
places of observation which require no emphasis. If growth
and vegetative increase are the outcome of preponderant ana-
bolism, reproduction and sexuality as their antitheses must re-
present the katabolic reaction from these. But anabolism and
katabolism are the two sides of protoplasmic life ; and the
major rhythms of the respective preponderance of these, give
the familiar antitheses we have been noting. These contrasts
of metabolism represent the swings of the organic see-saw;
the periodic contrasts correspond to alternate weightings or
lightenings of the two sides. Yet the contrast is less than it
seems. In previous chapters we have seen how growth, be-
coming overgrowth, turns into reproduction ; and how sexual
reproduction, dispensing with fertilisation, may degenerate till
we know it no longer from growth. Reproduction, moreover,
is as primitive as nutrition, for not only do hunger and love
become indistinguishable in that equal-sided conjugation which
has been curiously called " isophagy," but nutrition in turn is
nothing more than continual reproduction of the protoplasm.
Here, indeed, we have been anticipated by Hatschek, who
clearly states the more than verbal paradox, that all nutrition
is reproduction.
§ 7. The Contrast between Asexual and Sexual Repro-
duction, — In plenty, the hydra buds ; in poverty, it reproduces
sexually. In the same way, the liverwort on the flower-pot
bears its pretty cryptogamic "flowers" when its exuberant
growth and budding have come to an end. On rich soil a
plant has luxuriant foliage ; but great abundance is the reverse
of conducive to the richest crop of flowers and fruit Gruber,
Maupas, and others, have shown that abundant nutrition
favours the asexual multiplication, i.e., the division of in-
fusorians. In other words, the maximum size is rapidly
reached when food is abundant, but the conditions at the
limit of growth bring about reproduction. Preponderant ana-
bolism leads up to the possibility of multiplication, but we
need the onset of katabolism to bring about the reproductive
crisis. Gruber also notes, that in the very reverse of favour-
able conditions, rapid division with diminution of size and
resulting conjugation sets in; and Khawkine observes the
occurrence of division, both at an optimum and in famine.
In both cases a katabolic crisis is associated with reproduction,
though the crisis may be, and often is, preceded by an anabolic
preponderance.
16
949 THE EVOLUTION OP SEX.
In regard to a common infusorian (Jxucophrys patula\
Maupas observes that with abundant food the ordinary fission
continues, but with scanty nutrition a metamorphosis occurs,
followed by six successive divisions, which have for their end
conjugation. That is to say, we have positive proof that in
these lowest organisms, katabolic conditions determine the
beginning of sexual reproduction, a matter of no small import-
ance to the evolutionist Generalising, M. Maupas concludes
that the reproductive power of ciliated infusorians depends, (i)
on the quality and quantity of the food; (2) on the tempera-
ture; (3) on the alimentary adaptation of the buccal organs.
He also demonstrates that with a vegetarian diet their rate of
asexual reproduction is much less, and the size smaller. Taking
these facts, along with his important demonstration that the
life of ciliated infusorians runs in cycles of asexual reproduc-
tion, necessarily interrupted (if the life of the species is to
continue) by conjugation or sexual reproduction, we again reach
the general conclusion that anabolic conditions favour asexual
reproduction, rather than sexual; and that while preponderant
anabolism is the necessary condition of the overgrowth which
makes the asexual reproduction possible, the onset of katabolic
preponderance is necessary to the act itself.
Semper quotes an interesting observation by Strethill Wright,
unfortunately somewhat vague, that certain polyps multiply
abundantly in the dark by buds, while in the light, and with
insufficient supplies of food, they bring forth sexual individuals
or medusae. More precise is the fact already cited fror.}
Zacharias, that the spontaneous asexual multiplication of
planarians went on apace when the food supply was copious
(anabolic condition), but if the amount of food was reduced
or altogether withdrawn (katabolic condition) the asexual re-
production completely ceased. Bergendal reports that in the
transverse division of another planarian worm {Bipalium\ the
severed links were all sexually immature; and the results of
Rywosch demonstrate the same antithesis between the sexual
and the asexual process.
In the same way, sexual reproduction is contrasted with its
degenerate expression in parthenogenesis. The conditions of
the latter in aphides and phylloxera are demonstrably anabolic,
the normal sexual process recurs with the periodic return of hard
times, or in relatively katabolic conditions. In the lower crus-
taceans, a similar contrast of conditions has also been observed.
GROWTH AND REPRODUCTION. 243
It 15 again, on the present view, readily intelligible why in
the exceptionally favourable anabolic environment of bacteria
and many parasitic fungi sexual reproduction should be absent.
Marshall Ward has pointed out that the more intimate the
degree of parasitism or saprophytism, the more degenerate the
sexual reproduction. The greater the anabolism, in other
words, the more growth and the less sexuality. That such
comparatively complex organisms can continue their asexual
reproduction, dispensing altogether with the acknowledged
stimulus of fertilisation, may probably be at least partially
explained on the assumption that the abundant waste products
of the host act as extrinsic stimuli.
On this view, moreover, alternation of generations loses
much of its uniqueness. The contrast between the vegetative
asexual hydroid or hydra-tuba, and the active sexual medusoid
or jelly-fish, is very marked. So too, on a higher plane, the
vegetative spore-producing fern-plant stands opposed to the less
nutritive sexual prothallus. The alternation is but a rhythm of
lai^e amplitude between anabolic and katabolic preponderance.
What is so marked in the alternation is only a special-
isation of the reproductive or sexual parts of the organism as
against the growing or asexual ones, — a specialisation which
becomes ex^gerated into separate existences, each dominated
by its own physiolc^cal bias.
In the fern or flowering plant the vegetative or asexual
existence has preponderated, and this is entirely consistent
with the characteristic passivity of plants. This is emphatically
244 THE EVOLUTION OF SEX.
their line of development; but, be it observed, that though
in the flowering plants the nutritive generation has dwarfed,
and included the sexual, which seem indeed to be mere
organs, — the pollen -grain and embryo sac, — ^yet it is through
and for these that we have all the glory of the flower. In
animals, with their emphatically active line of development,
the reproductive generation is the higher; and in the higher
forms the separate asexual existence is wholly lost.
The experiments of Klebs may perhaps be regarded without
unfairness as marking the real beginning of a physiology of
reproduction in plants. For he has set himself to show how
deflnite environmental conditions of nutrition, temperature, &c.,
are definitely associated with the occurrence of particular
modes of reproduction in Algae and Fungi. A VaucAeria-pisint,
kept sterile for years, can be made sexual in a few days. A
form normally bisexual can be made unisexual. Asexual spore-
formation can be induced with certainty by one set of con-
ditions, e.g., in Hydrodictyon, and the appearance of sexual
gametes by another. Only by definite experiments like these
can we pass from vague interpretations to a precise physiology.
GROWTH AND REPRODUCTION. 245
SUMMARY.
X. Growth is characteristic of living organisms, though analogous pro-
cesses occur at the inorganic leveL Hunger is an essential characteristic of
living matter. As certain as the fact of growth, is the deiiniteness of its
limit alike for cell and for organism in the great majority of cases.
2. Spencer has analysed the limit of growth, in terms of the continual
tendency that increase of mass must have to outrun increase of surface.
3. Cell-division at the limit of growth, at the maximum or optimum of
size, restores the balance between mass and surface. The actual mechanics
of the process are at present beyond analysis.
4. Spencer's analysis may be restated in protoplasmic terms. Growth
expresses the preponderance of anabolism; increase of mass, with less
rapid iocrease of nutritive, respiratory, and excretory surface, involves a
relative predominance of katabolism. The limit of growth occurs when
katabolism has made up upon anal)olism, and tends to outstrip it. What
is true of the unit, applies also to the entire multicellular orgamsm.
5. Throughout organic life there is a contrast or rhythm between
growth and multiplication, between nutrition and reproduction, corre-
sponding to the fundamental organic see-saw between anabolism and
katabolism.
6. This contrast may be read in the distribution of organs, in the periods
of life, and in the different grades of reproduction. Yet nutrition and
reproduction are fundamentally nearly akin.
7. The contrasts between continuous growth and discontinuous multi-
plication, between asexual and sexual reproduction, between partheno-
genesis and sexuality, between alternating generations, are all different
expressions of the fundamental antithesis.
LITERATURE,
Drlagb, Y. — La Structure du Protoplasma, etc. Paris, 1895.
Klebs, G. — Ueber einige Probleme der Physiologie der Fortpflanzung.
Jena, 1 895, 26 pp.; and Die Bedingungen der Fortpflanzung bei
einigen Algen und Pilzen. Jena, 1896
SP£tNC£R, Principles of Biulogy ; and Hakckkl, Generelle Morphoiogie.
CHAPTER XVII.
Theory of Reproduction.
§ I. 7^ Essential Fact in Reproduction. — In the foregoing
chapters, the facts involved in the different forms of repro-
duction have been analysed apart, and separately discussed.
Male and female organisms have been interpreted as relatively
katabolic and anabolic; the origin of sex, in the individual
and in the race, has been traced back to the preponderance of
anabolic or katabolic conditions; the ultimate sex-elements
were seen to exhibit the same contrast in its most concentrated
expression ; fertilisation was regarded, not merely as a mingling
of hereditary characters, but as a katabolic stimulus to an
anabolic cell, and on the other side, of course, as an anabolic
renewal to a katabolic cell. Only by a separation of the
problem of "sexual reproduction" into its component problems
can clearness be reached. Sexual reproduction is like a
complex musical chord in the organic life, combining several
elements, all of which, however, admit of the same funda-
mental analysis. Two problems remain, — the psychical aspect
of the process ; and the import of that common feature of all
reproduction, the separation of part of the parent organism to
start a fresh life. The latter forms the subject of the present
chapter.
§ 2. Argument from the Beginnings of Reproduction. —
Leconte and others have pointed out that reproduction really
begins with the almost mechanical breakage of a unit mass of
living matter, which has grown too large for successful co-ordi-
nation. Reproduction, in fact, begins as rupture. Large
cells beginning to die, save their lives by sacrifice. Reproduc-
tion is literally a life-saving against the approach of death.
Whether it be almost random rupture or fragmentation, as
seen in some of the most primitive Protozoa, or the overflow
and separation of multiple buds as in Arcella^ the organism,
which is becoming exhausted, saves itself, and multiplies in
reproducing. In some cases, reproduction is effected by
thfidRV OF REt>R0DUCTIOll. ^4)
Outflowing processes of the cell, which have gone a little too
far. Now, such primitive forms of multiplication, gradually
becoming more definite^ express a relative predominance of
katabolism in the unit mass. Reproduction in its simplest
forms is associated with a katabolic crisis.
Nageli's hypothesis as to the origin of reproduction was (in brief
outline) as follows: — The original very simple organisms arose from a
natural synthesis of albuminoid molecules, a combination of these into
niicellse, and a union of these into protoplasts. But the balance between
nutrition and expenditure was at first very insecure, and external changes
frequently tended to induce latent life, or death. But in the very article
of death came the beginning of a new life. Itself too large for coherence,
the protoplast broke into parts which lived on, or in other conditions it
died down in great part but lived on in a fragment. As differentiation
proceeded, the first mode of reproduction by rupture gave place to cell-
division, bursting into cells^ discharge of special cells; the second gave
})lace to various forms of free-cell formation, spore-building, gemmule
brming, &c.
§ 3' Argument from Cell-Division. — Most unicellular
organisms reproduce by cell-division, and this is, of course, a
precedent of reproduction in multicellular organisms, whether
they multiply by asexual budding or by differentiated sex-
elements. But in the preceding chapter, following Spencer,
we have emphasised the connection between division and a
katabolic predominance within the cell. A constructive
period may precede, but a disruptive climax attends the
division. So far then as reproduction is either wholly included
in the process of cell-division, or has this as its necessary
precedent, it is associated with a katabolic crisis.
§ 4. Argument from the Gradations between Asexual
Severance of Parts and the Liberation of Special Sex- Cells. —
Discussing asexual reproduction, we have noticed that some
worm-types break into two or more parts, which start new
individuals. That some nemerteans normally break up into
pieces, as they do in the feverish anxiety of capture, is most
probable; and this is certainly the case in certain annelids.
From a syllid, which sets free a sexual individual, the over-
growth of an asexual parent, to one which liberates a series of
joints, or even a single joint, bearing reproductive elements, is
but a slight step. From the last case, to the rupture which
liberates sex-elements, is again only a slight advance. A
similar series is well illustrated among the Hydromedusse.
The breakage or thinning away which sets a large portion free
a^k THE EVOLUTION 6t SEJ£.
is a katabolic process, in a sense a local death. The gentle-
ness of the gradient warrants us in concluding that the
liberation or sex-cells, in its earlier expressions at least, is
associated with a local or with a general katabolic crisis.
§ 5- Arguntint from the Close Connection beltveen ReproduC'
lion and Z?m/A. ^Without going back to primitive disintegra-
tions, or the asexual severance of more or less large portions,
we may point further to the close connection between repro-
duction and death, even when the former is accomplished by
specialised sex-celts. We shall presently discuss at greater
length this nemesis of reproduction, but it is important here
to emphasise that the organism not unfrequently dies in
continuing the life of the species. In some species of the
tHEORY Of REPRODUCTION. 449
primitive annelid FolygordiuSy the mature females die in
liberating the ova. At a very different level, the gem mules of
the common fresh-water sponge are formed in the decay of
the asexual adult, while even the sexual summer forms,
especially the males, are peculiarly unstable and mortal The
whole history of this form seems a continuous rhythm between
life and growth on the one hand, and death and reproduction
on the other. Or again, the flowering of phanerogams is often
at once the climax of the life and the glory of death. In his
ingenious essay on the origin of death, Goette has well shown
how closely and necessarily bound together are the two facts
of reproduction and death, which may be both described as
katabolic crises.
§ 6. Argument from Environmental Condiiions which favour
Reproduction, — The rhythm between nutrition and reproduc-
tion, or between growth and multiplication, has been as it
were the refrain of the preceding pages. This " organic see-
saw " is determined by the very constitution of the organism ;
in other words, it expresses the fundamental characteristic of
living matter. It is an incomplete conception, however, unless
it be remembered that about this "organic see-saw" there
blows the wind of the environment, swaying it now to one
side, now to the other. It is important therefore to illustrate
how the play of external conditions accelerates or retards the
reproductive function.
The influence of heat upon the reproductive powers of
infusorians has been carefully investigated by Maupas. The
higher the temperature up to a certain limit, the faster do
these organisms reproduce. In favourable nutritive condi-
tions, Styhnichia pustulata divides once in twenty-four hours
at a temperature of 7* to 10* C, twice at 10' to 15', thrice at
15' to 20', four times at 20* to 24', and five times at 24*
to 2'f C. Illustrating the rapid rate of increase, Maupas
notes in the same paper, that at a temperature of 25* to 26'' C,
a single Stylonichia would in four days have a progeny of a
million, in six days of a billion, in seven and a half days of a
hundred billions I In six days the family would weigh one
kilogramme, and in seven and a half days one hundred
kilogrammes.
The action of heat may be twofold ; up to a certain limit
it quickens development and the general life, favouring asexual
reproduction and parthenogenesis rather than the sexual pro-
250 THE EVOLUTION OP SEX.
cess ; beyond that limit of comfortable warmth, so variable for
different animals, it may induce a feverish habit of body, and
hasten reproductive maturity and sexual reproduction. In
other words, heat may in some cases favour anabolism, in
others katabolism. It is intelligible enough to find increased
heat sometimes associated with increased asexual reproduction,
sometimes with accelerated sexuality. Instances of both may
be gathered from Semper's " Animal Life," the classical work
on the influence of the environment upon the organism.
Maupas supplies another vivid illustration of a yet more
important environmental influence, that of food. In another
ciliated infusorian {Leucophrys\ so long as food is abundant,
flssion obtains ; but when food grows scanty, there is a meta-
morphosis without encystation, followed by six successive
divisions. These are efiected, however, "without vegetative
growth, and have for their final object not multiplication but
conjugation.'' In other words, abundant food is associated
with asexual reproduction; a check to the nutrition brings
about the sexual process. Maupas gives a vivid numerical
statement of the stimulus to reproduction by a sudden check
to the nutrition. Leucophrys at a temperature of 20** C^ in
richly nutritive conditions, will give rise to sixteen thousand
three hundred and eighty-four individuals in three days ; but
if the food be then suppressed, this large number will in a few
hours be multiplied by sixty-four, resulting in a total of one
million forty-eight thousand Ave hundred and seventy-six
individuals !
From cases already cited, which may be multiplied by con-
sulting Semper's " Animal Life," supplemented by a summary
of more recent researches by one of ourselves, the general con-
clusions may be drawn, — {a) That heat increases reproduction,
either directly or as the result of a preliminary acceleration of
growth ; {b) That increased food will, of course, favour growth,
but reproduction may follow all the more markedly as an
exaggerated nemesis ; {c) That checks to nutrition, especially
in the form of sudden scarcity, will favour sexual reproduction.
The clearest result of all is, that a sudden katabolic change
favours reproduction, especially in its sexual form. Anabolic
conditions favour reproduction indirectly; the reverse condi-
tions have a direct influence ; in both cases, reproduction is
the expression of a katabolic crisis.
7. Conclusion, — Primitively, then, reproduction was a kata*
THEORY OP REPRODUCTION. 25 1
bolic rupture of a mass of protoplasm. This becomes more
definite in cell-division of various kinds, tending ever to occur
at the limit of growth when waste has made up on repair, or in
katabolic conditions due to the environment In multicellular
animals, anabolic conditions favour overgrowth; a check to
this brings about discontinuous asexual reproduction. With
increasing differentiation, the asexual multiplication is replaced
by the liberation of special sex-cells, by which the life-saving
and life-continuing sacrifice is rendered less costly. Just as
asexual reproduction occurs at the limit of growth, so a check
to the asexual process is often seen to involve the appearance
of the sexual, which is thus still further associated with kata-
bolic preponderance. This is confirmed by the contrasts
observed in alternation of generations, where the two processes
in varying degrees of distinctness persist in the life-history of
the same organism. Corroboration is again afforded by the
association of sexual reproduction with sundry environmental
checks of a katabolic character. And thus the opposition
between nutrition and reproduction, which, after life and
death, is the most obvious antithesis in nature, admits of
being more precisely restated in the thesis, that as a continued
surplus of anabolism involves growth, so a relative preponder-
ance of katabolism necessitates reproduction.
252 THE EVOLUTION OF SEX.
SUMMARY.
1. The essential fact in reproduction is the separation of part of the
parent organism to start a fresh life.
2. Reproduction begins with rupture, — a katabolic crisis.
3. Cell-division, which sometimes sums up, and is always associated
with, the act of reproduction, occurs at a katabolic crisis.
4. The gradations between discontinuous asexual multiplication and
ordinary sexual reproduction, show a lessening of the sacrifice; but all
demand a disruption, or a katabolic preponderance.
5 From first to last reproduction is linked to death.
6. Environmental conditions of a katabolic character favour sexual
reproduction.
7. General conclusion, — a relative preponderance of katabolism is
associated with reproduction, though tnis may be prepared for by a
previous period of predominant anaboUsm.
LITERATURE.
Geddes, p.— Theory of Growth, Reproduction, Sex, and Heredity. Proc
Roy. Soc. Edin., 1886.
I i A ECKRL. — Generelle Morphologie. 1 866.
Nagrli, C. Von. — Mechanisch-physiologische Theorie der Abstammungs-
lehre. MUnchen und Leipzig;, 1884, pp. xi.-822.
Skmper. — Animal Life. Int. Sci. Series. 1881.
Si'ENCRR. — Principles of Biology. 1866.
Thomson.— "Synthetic Summary of the Influence of the Environment
upon the Organism." Proc. Roy. Phys. Soc Edin., 1887.
CHAPTER XVIII.
Special Physiology of Sex and Reproduction.
It is no part of our purpose to discuss in detail the physiology
of sexual and reproductive functions. The fundamental physi-
ology of the essential functions has been the subject of preced-
ing chapters ; the details will be found in the standard works
on Physiology, Botany, and Zoology. For the sake of com-
pleteness, however, it is necessary to take a brief survey of
some of the most outstanding facts.
§ I. Weismann^s Theory of " Continuity of the Gemi-
Plasma'^ — Thanks, especially to Weismann, the view that
ordinary cells of the "body" become at a certain epoch
changed into special reproductive cells, may now be put aside
as exceedingly improbable. In a minority of cases, already
quoted, the reproductive cells, or the rudiments of sexual
organs, are demonstrably set apart at an early stage, before the
differentiation of the embryo has proceeded far. They thus
include some of the original capital of the fertilised parent
ovum intact, they continue the protoplasmic tradition un-
altered, and, when liberated in turn, they naturally enough
develop as the parent ovum did. Following out this important
fact, various naturalists have reached the conception of a con-
tinuous necklace-like chain of sex-cells from generation to
generation, — a continuous chain upon which the mortal indi-
vidual organisms arise and from which they drop away, like so
many separate and successive pendants.
But in the majority of cases, such a conception, as Weis-
mann has justly insisted, gives a false simplicity to the facts.
A chain of insulated sex-cells, connecting the parental fer-
tilised ovum with the germ-cells which develop into offspring
is, so far as we yet know, only rarely demonstrable. In other
words, the rudiments of the reproductive organs often appear
at a relatively late stage in the development. Where do they
come from? Are somatic, or ordinary body-cells modified
«54
THE KVOLUTION OF SBX,
into reproductive elements? Weismann's answer is a decided
negative. Although no continuous chain of germ-like cells is
demonstrable, there is a strict continuitjr of g^rm-plasm. To
quote Weismann's own words, " In each development a por-
tion of the specific germ-plasm which the parental ovum con-
tains, is not used up in the formation of ihe offspring, but is
reserved unchanged to form the germ-cells of the following
generation." In short, continuity is kept up by the plasm of
nuclei, rather than by a chain of cells. It will be observed, of
course, that while early insulation of definite germ-cells is a
Luidid Maga (s).— Afttr j>fit
demonstrable fact, to be seen in a few cases, though perhaps
of wider occurrence than we know of, the continuity of germ-
plasm Is strictly a hypothesis.
This being so, reproductive maturity may be defined as
the period when the reproductive cells (bearing the inherited
capital of germ-plasm) have established themselves to that
degree that they can start fresh organisms, and have multiplied
to an extent which in most cases makes their liberation a
physiological necessity. In the lower animals, the maturity of
the sexual functions is often as slightly marked as the liberation
SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 255
of the elements is passive and random. In slightly differen-
tiated organisms, like sponges, there is little reason to suppose
that the distinction between cells preponderating in germ-
plasm and the ordinary cells of the body is much marked.
Nor in such cases is the anarchic opposition between body
and reproductive cells at all emphatic, especially as regards the
female cells. It is only as the dififerentiation increases, as the
contrast between body-cells and sex-cells becomes emphasised,
as the asexual mode of getting rid of surplus wanes, that the
typical liberation of sex-elements which marks sexual maturity
becomes a striking fact in the life. That the male-cells are
always more anarchic, usually mature before the female ele-
ments, and even in plants, and in such passive animals as a
sponge or a hydra, burst from the organism, while the female
cells remain in situ^ is quite consonant with their relatively
katabolic character.
§ 2. Sexual Maturation. — ^The maturation of the sexes not
only acquires increasing definiteness in the higher forms, but
becomes associated with various characteristic accompaniments.
The profound reaction of reproductive maturity upon the whole
system is best marked in birds and mammals, and perhaps
most of all in man. Thus in a young male bird, the circulation
in the testes is greatly increased, and these organs increase
greatly in size and weight, and commence to develop sperma-
tozoa. Meanwhile the '* secondary sexual characters'' of the
adult — gayer plumage possibly attractive to the female, or
weapons for contest with other males — appear, the voice and
note may alter, and a marked increase of strength and courage
may occur. Among mammals, the changes are of similar
order, the secondary sexual characters of course differing in
detail. The minor changes at puberty in man associated with
the commencement of spermatogenesis, are (besides the reflex
excitation of erection due to distension of the seminal vesicles,
and the more or less periodic expulsion of their contents during
sleep) the growth of hair on the pubic region and later on the
lower part of the face, and the rapid modification of the
laryngeal cartilages and the lengthening of the vocal chords, so
rendering the voice harsh and broken during the change, and
ultimately deepening it by about an octave. The marked
strengthening of bones and muscles, and the profound psychical
changes which accompany the whole series of processes, are
also familiar.
256 THE EVOLUTION OP SEX,
Although we do not understand the facts, there is no doubt as to the
profound correlation which exists between the reproductive organs and the
Dody. This may be illustrated in various ways, ^.^., by reference to the
effects of castration. On young cocks, according to Sellheim, this opera-
tion has very diverse results, sometimes decreasing the secondary sex-
characters, and sometimes, strange to say, increasing them. The capons
seldom crow, or do so abnormally; attempts at copulation are rare. But
one general result seems to be established, that the whole body is
affected: — the larynx is intermediate in size between that of cock and
hen ; the syrinx is weakly developed ; fat accumulates in the connective
tissue ; the brain and heart are light in weight ; even the skeleton shows
many abnormalities. ("Beitrage zur Geburtshilfe und Gynakologie,'' i.,
1898, PJ3. 229-246.) The same investigator has also shown that in mam-
mals of both sexes the removal of the reproductive organs markedly pro-
longs the period of bone-development, and has thus an effect on the shape
of various parts, {flp, ciU^ ii., 1899, pp. 236-59.)
In regard to the correlation of antlers and reproductive organs ('* Arch.
Entwickmech.,'* viii., 1899, pp. 382-447), A. Rorig has investigated five
points. His first question is : Does the absence of antlers or the development
of only one depend on an abnormality of the reproductive system ? He
answers that the condition may occur with both normal and abnormal
gonads. His second question is: Can the occasional development of
antlers in female Cervidse be referred to the abnormal development of the
reproductive organs ? He answers that a diseased state on one side may
be correlated with the development of one antler, on both sides with the
development of two antlers, and that one-sided disease has a correlation
operating transversely. If the ovaries are atrophied there are usually
antlers. Hermaphrodites seem always to have antlers, and these are the
more perfect the more the gonads incline towards maleness. Irritation of
the appropriate place may also evoke antlers in females. His third
question is: What effect has partial or total castration in the males? It is
answered that the effect vanes according to the age of the animal and the
stage of antler development. In a young quite hornless male, castration
entirely inhibits the growth of antlers. Fourthly, Rorig points out that
atrophy of the testes is followed almost always by the formation of
'* Per iicken "-antlers, and injury to the testes by premature casting.
Fifthly, the excision of the antlers has no deleterious effect on the repro-
ductivity or health of the individual. It is obvious that this is a very
important contribution to our knowledge of the correlation between gonads
and soma.
In higher vertebrates, the sexual maturity of the female is
marked by a cellular activity within the ovary, not less remark-
able than that in the testes. Associated therewith are minor
but often very important characteristics, such as the increased
mammary development in mammalia. In some of the lower
animals, such as certain marine annelids, the ova become so
numerous that their disruption or liberation is in great part a
mechanical necessity. The same might be said of fishes,
reptiles, and birds. At the same time the enlargement and
SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 257
escape of the ova are doubtless expressions of a normal cellular
rhythm, of which hints are given in the frequent passage from
an amoeboid to an encysted phase, in the occasional relapse to
the former, and in the fatty degeneration or death of ova which
have not accomplished their destiny.
The primitive ova of vertebrates lie in clusters in the substance or
stroma of the organ, and are produced from the essential germinal
epithelium. Only a minority, however, grow into genuine ova ; others, of
smaller size, form a nutritive sheath or follicle around them. In mammals,
each follicle forms a cavity containing a fluid. Into this the ovum, sur-
rounded by a mass of follicle cells, projects. When mature, the follicle with
its contained ovum has attained a superficial position. By the bursting of
the ripe follicle the ovum is expelled, and passes into the approximated and
ciliated upper end of the oviduct or Fallopian tube. The rupture of
blood-vessels in the substance of the ovary fills up the Graafian follicle with
blood. The white corpuscles form a framework resembling connective
tissue, in which the solids and corpuscles of the blood serum, with colour-
ing matter derived from the haemoglobin of the latter, are retained. The
whole constitutes the *' corpus luteum," which, should pregnancy occur,
may persist and undergo further retrogressive changes, or otherwise
gradually disappear.
As to the direct causes of this process of ovulation there is some
difference of opinion. The congestion of the blood-vessels of the ovary,
its own internal turgidity, a slight contractility of its stroma, have been
regarded as determining factors. The process seems, however, rather to
depend upon the growth and turgescence of the individual follicle. The
question of the relation of ovulation to the process of copulation in the
higher animals has also been much discussed. Though we certainly know
that ovulation is of regular occurrence whether fecundation takes place or
not, it seems that in many cases copulation is speedily followed by the
liberation of an ovum ; nor is it difficult to see how the profound nervous
and circulatory excitement associated with the former process might
accelerate the bursting of a follicle. Leopold has conclusively shown,
however, that ovulation may also long precede impregnation, and Stratz
has shown, in the case of Tupaia javanica^ that after fertilisation of ova shed
into the oviduct, the follicles in the ovary undergo degeneration, and no
more mature eggs are formed till near the end of pregnancy (" Die Gesch-
lechtsreife Sailgethierstock." Haag, 1898, pp. 67, 9 pis.). The experi-
ments of Heape (" Proc. Roy. Soc," Ixi., 1897, pp. 52-63) and others
show that artificial insemination may be effective in certain mammals, such as
mice and dogs; on the other hand, Heape's observations on the rabbit point
to the conclusion that both copulation and the presence of spermatozoa in
the uterus are necessary to induce ovulation in the virgin rabbit when she
is in ** heat."
Since the oviduct, unlike its male counterpart, is not, in the vast
majority of vertebrates, continuous with its associated organ, it is often
dimcult to see how the ova once liberated into the body-cavity find their
way safely into the small opening of the duct. The problem has been
worked out by Nussbaum in the frog, where the ova are moved by the
effective action of numerous muscles and by the ciliary activity of peritoneal
17
2$S THE EVOLUTION OF SEX.
epithelial cells which are disposed in tracts converging to the oviducal
aperture, so propelling the ova in the right direction. In reptiles, birds,
and mammals the open end of the oviduct is widened, fringed, and
ciliated, and lies close to or even touching the ovary; muscular fibres too
are present, and more or less active movements of this ciliated end over
the ovarian surface have l>een alleged to occur. The oviduct once reached,
the downward progress of the ovum is ensured by the cilia of the epithelial
lining, and probably also by peristaltic movements of its muscular coat.
There is no doubt that the advent of sexual maturity varies
with environmental conditions of climate, food, and the like.
Broadly speaking, sexuality becomes pronounced as growth
ceases. Especially in higher organisms, a distinction must
obviously be drawn between the period at which it is possible
for males and females to unite in fertile sexual union, and the
period at which such union will naturally occur or will result in
the fittest offspring. In the lower animals, where the individual
life is usually shorter, sexual maturity is more rapidly attained,
though we find cases such as that of the fluke (Fofysfomum) so
commonly present in the bladder of the frog, where maturity of
the reproductive organs does not occur for several (three) years,
and maturity of growth for some years afterwards. In cestode
parasites, the bladder-worm stage remains indefinitely asexual
until in fact the stimulus of a new host admits of the develop-
ment of the sexual tapeworm. In plants, reproductive maturity
sets in at various ages; thus we have all gradations, at the
one extreme our characteristically short-lived but magnificent
annuals, then the biennials, and from these to a maturation at
still longer date, as in the well-known case of the American
aloe {A/og atfiericana\ which even in Mexico takes from seven
to twelve years to reach the floral climax in which it expires,
and in our greenhouses as much as a generation or two, whence
its name of " century plant."
In contrast to such cases, precocious reproductive maturity
occasionally occurs. We have already referred to those
dipterous midges {Cecidomyia\ in which the larvse for succes-
sive generations become reproductive, though only partheno-
genetically. Very striking too is the trematode worm Gyro-
dactyluSy which recalls the mystical views of the preformation-
ists, in exhibiting three generations of embryos, one within the
other, while the oldest is yet unborn. The well-known axolotl
of Mexican lakes, though with its persistent gills in a sense
the larval form of Arnblysioma, attains of course to sexual
maturity. A more marked precocity has been observed in the
SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 259
Alpine salamander {Triton alpestris). In higher organisms, it
occasionally happens that long before growth has ceased or
adolescence been reached sexuality sets in, especially in the
male sex, but this is fortunately a comparatively rare patho-
logical occurrence. In one set of organisms precocious repro-
ductive maturity has been of paramount importance, viz., in
the flowering plants. Here the prothallus stage, as contrasted
with the vegetative, has been much reduced, and has remained
associated with or been absorbed by the asexual generation.
This is to be in part explained by the accelerated reproduction
of the prothallus, comparable to a similar process which has
reduced the separate medusoid sexual persons of a hydroid
colony to mere buds.
It would be interesting to consider in this connection the
profound changes of habit often associated with reproductive
maturity, but in most cases the facts are inadequately known.
It has been often said that it is a nisus generaiivus which
prompts the salmon to leave the sea where it feeds and to
pass up the rivers where it reproduces, but it must be noted
that salmon are seen ascending the rivers throughout the whole
year in all stages of reproductive development (D. Noel Paton,
Report on Life History of Salmon. Fishery Board for Scotland,
1898). It has been often said that it is a nisus generativus
which prompts the migratory birds to fly in spring from the
warmer areas where they winter to the colder areas where they
breed, and it is admitted that the adult males are the flrst to
leave, but the difiiculty remains that a large proportion of the
migrants are in most cases immature, and do not breed in that
season.
§ 3. Menstruation, — The process of menstruation {menses^ caiamenia)^
although from the earliest times the subject of medical inquiry, is by no
means yet clearly understood. It occurs usually at intervals of a lunar
month in all women during their period of potential fertility (fecundity), and
is not confined to the human species, having been observed in a number of
mammals, tf.^., some monkeys {Macacus rhesust Semnopitkecus entellus),
the lemur Tarstus, Tupaia javanica^ and the common shrew (in the last
case without proof of outflow of blood). See W. Heape, " Proc. Roy.
Soc., London," Ix., pp. 202-205.
Though thus clearly a normal physiological process, it yet evidently lies
on the borders of pathological change, as is evidenced not only by the pain
which so frequently accompanies it, and the local and constitutional disorders
which so frequently arise in this connection, but by the general systemic
disturbance and local histological changes of which the discharge is merely
the outward expression and result. In general terms, and apart from
26o THE EVOLUTION OF SEX.
ovulation, menstruation may be described as a periodic discharge of blood,
glandular secretion, and cellular detritus from the lining of the uterus.
After from three to six days the blood ceases to appear, and the lost
epithelium is rapidly replaced, apparently by proliferation from the necks
of the glands. By the ninth or tenth day the mucous coat is fully healed,
and the beginnings of the next menstrual process recommence.
The age at which the process commences varies with race and climate,
with nutrition and growth, with habit of life (^.f., with difference between
town and country life), and with mental and moral characteristics. Of
these, however, climate seems most important; thus, while in Northern
Europe the age is reckoned at the l)eginning of the fifteenth year, in the
tropics it commences earlier, in the ninth or tenth year, according to some.
The cessation of menstruation usually takes place between the age of forty-
five and fifty, and, somewhat as the secondary characteristics of female
puberty coincide with its appearance, a less distinct reduction of these is
associated with its close; in many cases secondary resemblances to the
masculine tvpe may supervene.
The old theories of menstruation were, that it served to rid the system
of impure blood, that it simply corresponded to the period of "heat"
observed in lower animals, or, later, that it was associated with ovulation,
— which indeed seems broadly to correspond with the end of the menstrual
period. In Tupaia^ Stratz has shown that the eggs are mature at the
beginning of menstruation, and ready for fertilisation at its end. And
while it cannot be maintained that either '* heat " or ovulation are neces'
sarily associated with menstruation in Homo^ there can be little doubt of
the general physiological parallelism of all three processes. At present
there may be said to be two rival theories. According to the first of
these, the process is viewed as a kind of surgical *' freshening *' of the uterus
for the reception of the ovum, whereby the fatter during the healing process
can be attached safely to the uterine wall. The other view is exactly the
reverse of this. Its upholders regard the growth of the mucous coat before
this commencement of the flow as a preparation for the reception of an
ovum if duly fertilised, and the menstrual process itself as the expression of
the failure of these preparations, — in short, as a consequence of the non-
occurrence of pregnancy. A decided majority of gynaecologists appear to
incline to the latter view.
The process may, however, be expressed in more general, and at the
same time more fundamental terms. If the female sex l)e indeed pre*
ponderatingly anabolic, we should expect this to show itself in distinctive
functions. Menstruation is one of these, and is interpretable as a means
of getting rid of the anabolic surplus, in alisence of its consumption by the
development of offspring, — just as it is intelligible that the process should
stop after fertilisation, when replaced by the demands of the practically
parasitic fcetus. In the same way, the occurrence of lactation, after this
internal parasitism has been terminated by birth, is seen to be reasonable.
The young mammal is thus enabled to become what is practically a
temporary ecto- parasite upon the unfailing maternal anabolic surplus ; and
when lactation finally ceases, we have the return of menstruation, from
which the whole cycle may start anew. So in the widely different yet
deeply similar vorld of flowers, the distinctly anabolic overflow of nectar
ceases at fertilisation, and the surplus of continued preponderant anabolism
is diafted into the growing seed or fruit.
SPECIAL PHYSIOLOGY OK SEX AND REPRODUCTION. 26 1
§ 4. Sexual Union, — In a previous chapter we have noted
the passive and random way in which the sex-elements of
many of the lower animals are liberated, and the chance
manner in which they are often brought together by water-
currents and the like, though this may not be quite so common
as our ignorance leads us to suppose, witness the alleged
occurrence of sexual union in Asterina and Antedon. Yet
more in plants is the liberation of male elements, and notably
that of pollen-grains, a passive dehiscence, and fertilisation a
matter of chance, only reduced by the prodigal wealth of
material. Secure as the methods of fertilisation of flowers
by the aid of insects often are, the margin of risk is wide ; and
this is yet more marked when the pollen is carried by the
wind. It is true that, both in plants and animals, there are
subtle attractions between the essential elements, but this is
only at a close range; and the external union is in many
cases none the less random.
It is important to keep clearly in mind the different forms
of sexual relation which may occur. There may be mating
at random within the limits of the race ("pangamic" mating),
or there may be casual mating between members of different
races, or there may be some form of selective or prefer-
ential mating within a race. Various forms of the latter
may be distinguished (following Pearson's classification and
terminology) —
{a) Autogamic, or self-fertilisation.
{£) Endogamic, within the family, brood, or clan.
If) Homogamic or assortative, or the mating of like with
like, the two mates not being of the same breed, or
not necessarily so.
{d) Preferential or apolegamic — /.^., with sexual selection
in the narrower sense of Darwin.
{e) Heterogamic, or the mating of unlikes. (See Pearson's
"Grammar of Science," 2nd ed., pp. 423-424.)
As it is essential that there should be a timely encounter
of the ovum and spermatozoon, we naturally find a very varied
series of adaptations securing fecundation. And while the
physical adaptations are important, we have also to recognise
that the increasing differentiation of the sexes has in the
higher animals been enhanced by psychical as well as physical
attractions, thus more and more ensuring the continuance of
the species.
a6a THE EVOLUTION OF SEX.
A not unfrequenl motle of fecuntiaiion is by means of spernialophon
or [Kickels of spetmatoioa. These may be seen at limes aKached to l
earth'Wonn, or found within the leech and snail. Even in newls spein
tophores may be formed, and taken up u such by the females.
In the ipidec the spetmUoioa aie stored in a special receptacle on t
UalaiirPapsNauEUiuM'XEjuiiAlX with il* oodified
patp, and hence hastily transferred to the fierce female. In cuttlefishes
this mode of impregnation is yet more marked. One of Ibe "arms" of
the male, much modilied and laden with spermatopbores, is Ihiust, or iq
many cases bodily discharged into the branchial cavity of the female, where
it bursts. Such a discharged arm was, on first discovery, regarded as a
'^™edfrom Iht urion of l»"ISdn"
(Di/crfa^il an Mrly !»£« in iheir life.
parasite, and hence received the name of Heclocotylus. A cunous aberra-
tion from the ordinary relations is figured above, where two distinct
individuals of a species of fluke (Difi/eiopn) join in almost life-long union.
In many cases again, especially in bony fishes, (here is a sexual
attraction between male and female, but wiihout any copulation. The
female, accompanied by her mate, deposits ova, which he thereupon
SPECIAL PHYSlOLOGV Ot SEX AWt) RftPRODUCtlON. 263
fertilises with spermatozoa. A slightly more advanced stage is seen in the
frog. Fertilisation is still outside the body of the mother, but the male,
embracing the female, liberates spermatozoa upon the eggs, just as these
are laid.
In the majority of cases, however, special organs for emitting and for
receiving spermatozoa are developed, and copulation occurs. The male
organ is often an adaptation of some structure already existing, as in many
crustaceans, where modified appendages form external canals for the
seminal fluid. In skates and other gristly fishes, the remarkably complex
copulatory organs, the so-called ''claspers," are in close connection with the
hind limb. The penis of higher vertebrates is virtually a new organ. The
copulation may be quite external, as in crayfishes, &c., where the male,
seizing the female, deposits spermatozoa upon the already laid eggs.
Oflener, however, it b internal, and the intromittent organ is inserted into
the genital aperture of the female. True copulation may occur without
the presence of special organs, — notably in the case of many birds, where
the cloaca of the male is apposed to that of the female. The spermatozoa,
forcibly expelled by the excited male organs, pass up the female ducts,
probably, in part, as the result of peristalsis, but chiefly at least by their
own locomotor energy, and one of them may eventually fertilise an ovum.
In addition to the intromittent organ, and the lower portion of the female
duct which receives it during copulation, there may be auxiliary structures,
such as true claspers for retaining hold of the females. The limy " cupid's
dart" or "spiculum amoris*' of the snail, is usually interpreted as a
preliminary excitant.
Three further notes in regard to higher animals are requisite, (i.)
There is much reason to believe that the follicles tend to burst towards the
end of menstruation ; that this may be accelerated by copulation ; successful
fertilisation may occur at any period, but most frequently soon after
menstruation, and most rarely during the relatively infertile period most
distant from that process. (2.) After conception, when the fertilised ^g
has begun to develop, the mouth of the uterus is closed by a secretion,
which prevents the entrance of other spermatozoa should further copulation
occur. (3.) The period of gestation — i.e., between the fertilisation of the
ovum and the extrusion of the foetus, varies widely in mammals, from
about 18 days in opossum, or 30 in rabbit, to about 280 days in Homo or
600 in the elephant, being longer in the more highly evolved types. But
the length of the period should also be considered in relation to size,
being about 280 days in cow and 150 in sheep; in relation to number of
oflspring, being about 350 in mare and 60 in dog ; and in relation to the
degree of maturity at birth) being 420 in giraffe and 40 in kangaroo.
§ 5. Farturiiion. — In many cases — e.g.y marine annelids,
mature ova burst, as we have already noted, from the mother
animal, who may thenceforth have nothing more to do with
them. Liberation of ova from the ovary and from the
organism may be almost coincident, as in most bony fishes.
In other cases, the ova are retained within the mother until
fertilised, but are expelled not long after, before development
has advanced to any marked degree. Such eggs are often
264 THE EVOLtJTlON OF SEJL
furnished with the important capital of nutriment, so familiar
in the case of birds, and may be also surrounded by chitinous,
horny, membranous, or limy shells. All such forms of birth
are familiarly described as oviparous.
In numerous invertebrates, fishes, amphibians, and reptiles,
the ova develop within the mother, and the young are born
more or less actively alive. To such cases, where there is no
nutritive connection between parent and offspring, the term
ovo-viviparous used to be applied. They were contrasted with
oviparous birth, as in birds, on the one hand, and with the
viviparous birth of mammals, on the other. It is the well-
known characteristic of the latter that there is an intimate
nutritive connection between mother and offspring. The term
is of little use, however, for the cases to which it is applied
shade off towards the two other forms of birth. Thus among
gristly fishes {Muste/us iavis and Carcharias\ in the curious
bony fish Anabieps^ and in certain lizards {Trachydosaurus and
6ycIodu5\ a somewhat placenta-like function is discharged by
the yolk-sac and the wall of the oviduct; while in fishes,
reptiles, &c., oviparous and ovo-viviparous birth may occur in
nearly related forms. The distinction involved in the term is
therefore abandoned, and it must also be recognised that the
difference between egg-laying and the production of young
actively alive is only one of degree. Even in mammals, which
are viviparous par excellence^ the two lowest genera — the duck-
mole and the Echidna — are oviparous. The common grass-
snake, normally oviparous, has been induced, in artificial
conditions, to bring forth its young alive, and this is probably
true of other forms. The parthenogenetic generations of
aphides are usually viviparous, while the fertilised eggs are laid
as such.
Beard has maintained that there is a definite stage in
development when the embryo first begins to put on its specific
characters, and calls this the critical stage. He believes that
in the early days of mammalian evolution, before an allantoic
placenta had arisen, the birth period and the critical period
(on to the critical stage) coincided ; and this is still seen in
several marsupials. Then, too, the ovulation period must
have been almost equal to — really a little longer than — the
critical period, for a coming ovulation, with its reflex message
from ovary to uterus, was the direct cause of birth. In higher
mammals, the evolution of an allantoic placenta provided for
SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 265
the nutrition of the foetus beyond the critical period, or, more
technically, beyond a single "critical unit" But the interesting
fact is that a correspondence between the length of gestation
and a certain number of critical and ovulation units (up to
eight) is still preserved. The critical period, multiples of this,
and the ovulation periods, must very frequently be times of
abortion in mammals; and menstruation is comparable to
an abortion prior to a new ovulation. There are no doubt
difficulties connected with this theory, and Beard ingeniously
deals with some of these, but the idea is a luminous one that
the span of gestation, the ovulation period, and the critical unit
are all connected as expressions of the rhythm of reproduction
in mammals, — a rhythm which has its basis in the ovary. By
ovulation the rhythm is proclaimed throughout the reproduc-
tive life of the female; in gestation the same rhythm is
maintained but in a modified fashion; and as the span of
uterine life draws to a close, it again asserts itself, and induces
birth. " Thus harmony and law reign in the reproductive life
of Mammalia."
§ 6. Early Nutrition, — The early nutrition of the embryo,
and even larva, is in most cases an absorption of the legacy of
yolk material, which is probably richest in the eggs of birds.
The tadpole of the frog grows and exerts itself for a short time
at the expense of its legacy; it then begins to feed for itself, but
it is interesting to notice that before metamorphosis is accom-
plished the growth of new structures appears to be provided
for by the nutritive absorption of the tail, the larva literally
living upon itself. The same is true in the elaborate meta-
morphosis of echinoderm larvae. In many cases, the cells of
the embryo, independently and actively, devour the yolk and
other available material, after the amoeboid fashion technically
known as intra-cellular. At the same time, osmotic currents
may more passively effect the like result.
In the buckie or whelk (Buccinum undatum) the eggs are
enclosed in capsules secreted from the sole-gland of the foot,
and the early nutrition is remarkable : a cannibalism occurs
among the crowd of embryos enclosed within each capsule.
The stronger and older devour the younger and weaker, — a
struggle for existence happily of exceptional precociousness.
The conception of a struggle for existence applies to the
earliest chapters of life. There is struggle among potential
ova, and struggle amid the crowd of spermatozoa; there is
266 THE EVOLUTION OP SEX.
Struggle between embryo and mother, and struggle between
adjacent embryos. Thus De Bruyne notes in regard to fresh-
water mussels, the struggle between the successful ova and
the adjacent cells, and the continuance of this between the
embryos and the maternal leucocytes, and even on to the time
when the larvae are temporarily parasitic in the skin of a fish.
(See "Arch. Biol.," xv., 1898, pp. 181-300, 5 plates.)
In the higher vertebrates (above amphibians), two foetal
membranes — amnion and allantois — are developed, in addition
to the yolk-sac which encloses the yolk. Of these the amnion
is mainly protective, and the allantois at first almost wholly
respiratory. But in birds (and to a slight extent in some
reptiles) the allantois begins to assume nutritive functions,
assisting in the absorption of the yolk. In placental mammals
this nutritive function becomes paramount, the allantois forming
the greater part of the embryonic side of the placenta. The
yolk-sac is here virtually yolk-less, but in some forms it serves
for a time to absorb nutriment as it did in birds, though from
a different source, — the maternal wall In most cases, how-
ever, what was incipient on the part of the yolk-sac in the
exceptional elasmobranchs and lizards already mentioned,
becomes the emphatic function of the allantois, — namely, the
establishment of a vascular or nutritive connection with the
wall of the maternal uterus. By this means a very intimate
osmotic transfusion is effected.
§ 7. Ldctation, — If menstruation be a means of getting rid
of anabolic surplus, in absence of the foetal consumption, lacta-
tion is still more an anabolic overflow, adapted to, though not
of course originally caused by the offspring's demands. It is
at the same time evident enough, and easily verified by the
histologist, that in actual occurrence both processes are kata-
bolic, involving cellular disruption and death. That peculiar
liability of these uterine and mammary tissues to disease,
which furnishes the most tragic possibilities of the life of
woman, becomes thus less mysterious. We can understand
more readily the association of such diseases with much of
what we are pleased to generalise as civilisation, and view
more hopefully the possibilities of their enormous diminution
by the rational hygiene of civilisation properly so called.
The milk or mammary organs are modified skin-glands,
probably most nearly allied to the ordinary sebaceous type,
except in monotremes, where they seem to be allied rather to
SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 267
the sweat glands. Every one knows that they are exclusively
characteristic of mammals, and are only normally functional in
the female sex. Rudimentary in the males, they may even
there produce milk (•* witches* milk ") at birth, puberty, and
under pathological conditions, while cases have been put on
record of males who have actually given suck. Merriam
(Hayden's U.S. Geol. Survey, VI., p. 666) gives a definite
account of male lactation in Lepus bairdi. They vary greatly
in position and number, a large number being doubtless
the primitive condition. It has also been shown that the
nuclei of the gland cells undergo degeneration, disruption, and
expulsion, and they, in all likelihood, form the casein elements
of the nutritive fluid. Some of the abundant leucocytes or
white blood corpuscles also migrate through the epithelium,
but it is not proved that they actually share in forming the
milk, though they may bring fat globules into it. (See L.
Michaelis, '* Arch. Mikr. Anat.," li., 1898, pp. 711-747, 2 pis.)
Before birth, the mammalian embryo has been nourished
through the placenta, by the transfusion already referred to.
The alimentary canal has obviously had no experience in
digestive function. Before it proceeds to digest the food of the
parents, it is put through a course of what Sollas neatly terms
'Agastric education,*' by feeding upon the readily assimilated
mother's milk.
§ 8. Other Secretions, — Every one has at least heard of
" pigeon's milk," and many are familiar with its administration
to the young birds. It is produced by both sexes, especially
just after the hatching of the young, and is the result of a
degeneration of the cells lining the crop. Some of the cells
break up, others are discharged bodily. The result forms a
milky emulsion-like fluid, which is regurgitated by the parents
into the mouth of the young bird. A similar substance is said
to occur in some parrots.
Of some interest also is the supra-salivation which occurs
at the breeding season in the swiftlets {Coilocaiia\ which form
the edible birds' nests, the costly, though to us wofully insipid,
luxury of Chinese epicures. Certain salivary glands become
peculiarly active in these birds when breeding, and the secre-
tion, which, according to Green, consists chiefly of a substance
akin to mucin, is used to form the snow-white fibrous nest.
Take only one other instance of peculiar secretion, curiously
linked to the above by one of those profound physiological
263 THE EVOLUTION OF SEX.
unities which show how superficial after all are the utmost
contrasts of organic form, — we refer to the viscid threads with
which the male stickleback weaves his nest. Mobius has
shown that the kidneys are greatly affected by the growth
of the testes ; that they produce, by a semi -pathological pro-
cess, special waste or katabolic elements, in the form of
mucous threads. The male gets rid of this uneasy encum-
brance (which has a somewhat parallel pathological equivalent
in higher animals), by rubbing itself against objects, and thus
almost mechanically has been evolved the familiar weaving of
the aquatic nest.
Tfai Ncuof lbeStickle1iick<C^/mu/nu).— FrflmTbOBiuBollDn
§ 9. Incubation. — The physiological sacrifice of the female
birds does not end with providing the large capital of nutritive
material with which the germ is endowed, but is continued in
all the patience of brooding. In passerine birds the male
relieves the female in her task of love, and in the ostrich and
Rhea he plays a prominent part In the cuckoos and cow-
birds the parental care is shirked, and with varying degrees of
deliberateness the eggs are foisted into foster nests, and the
young thus put out to nurse. After the fatigue of reproduction
it is perhaps natural enough that the female should rest awhile
upon the eggs in the shelter of the nest, and since there is
SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION, ^69
observed to be an increased circulation in the skin of the
abdominal region at this time, it has been argued that the bird
merely sits to cool itselfl Primitively, the incubatory instinct
may be traced back to the need for rest, as a reaction after the
fatigue of reproduction.
Here too one must include the retention of the young in
skin pouches, exhibited by the great majority of marsupial
mammab and by the Echidna. In the latter, the pouch is a
Tbc female Suriiun Twd, with young ana on in bacli.— Fiom Leiinii.
simple and possibly periodic structure, arising from an insink-
ing of the skin in the mammary region of the abdomen. Here
the eggs are somehow or other stowed away and the young
developed. The milk glands simply open on the surface of
the depression. In most marsupiala, the young, which are
born precociously after a very short uterine li'e, are sheltered
in similar, but more developed, pouches of the skin, within
which the teats open.
170 THE EVOLUTION OF SEX.
Id oviparous reptiles, the eggs are usually left to hatch of
themselves, aided by the warmth of sun and soil. "The
female python disposes herself in coils round her eggs, and
incutiates them for a prolonged period, during which the
temperature has been observed to rise as high aa 96* F. within
the coils."
Some exceedingly curious parental adaptations occur among
amphibians, which seem to have made numerous experiments
in this direction. Thus in the Surinam io&.d(Pipa), the male
spreads the ova on the female's back, a sort of erysipelas sets
The female KtMrtma manKiia/um,—^ amphiUin, wilh
egp !n A dar&.il uc, whLca \s shown partly micoveied.
in, and each ovum becomes surrounded by a skin-cavity in
which the t.idpole develops. After the process is over, the
skin of the b.ick is renewed. In other cases this mode of
carrying the ova becomes somewhat more definite; thus in
Nolodelphys and Nolotrema the eggs are stored in dorsal
pouches. Nor are the males without their share in the task
of parentage. In the obstetric frog {Alytes obslttrUani), the
male helps to remove the eggs from the female, twists them in
strings round his hind legs, and buries himself in the water till
the tadpoles escape and relieve him of his burden. In Hhino-
SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 27I
derma daraiinii, the croaking sacs, which were previously used
for amatory calling, become enlarged as cradles for the young.
In a Japanese tree-frog {Rhacophorus tchltgtlii) the female
The Su-hone l,HI*tKamivt rill*lalia\—fxati, tb«
Allu oTlhe Nspla Aqusrium.
lays the eggs in a hole in the mud, and by curious kneading
and treading movements works the jelly which surrounds them
into a froth. The frothy envelope is like well-beaten white of
Tbi rcnuk o( the " Pnpei Niudlu)" (.<r7i»wW<>aiTa), wiihlu
brood-chAoilier, — After Lcunis,
egg, and the outer surface dries into a crust. It protects the
eggs, and perhaps prevents overcrowding, but is of especial
service in facilitating the respiration of the eggs and embryos.
272 THE EVOLUTION OF SEX,
(S. Ikeda, "Annot Zool. Japon.," i., 1897, pp. 1 13-122, 2
figs.)
Among fishes, parental care is largely in abeyance, and
there are only slight hints of anything in the way of incuba-
tion. In a siluroid fish (A5predo\ the female deposits her ova
and lies upon them till they become attached to the spongy
skin of the belly, very much as happens in the dorsal attach-
ment of the Surinam toad. After hatching, the skin excres-
cence is smoothed away. In Soienostoma (allied to pipe-fish)
the ventral fins unite with the skin to form a pouch in which
the eggs are retained. In other cases, it is the male which
incubates or cares for the ova. Not a few form nests, as in the
stickleback, over which they keep a jealous guard. In some
species of Arius the eggs are carried about in the pharynx \
while in the sea-horses a pouch is developed on the posterior
abdomen.
Among invertebrates, brood-chambers or cradles for the
young are not uncommon. The capsules of hydroids, the
tent of spines on a few sea-urchins, the depressions in the skin
in one or two sea-cucumbers, the modified tentacles of some
marine annelids, the dorsal shell-chamber in water-fleas, the
incurved abdomen of higher crustaceans, the gill-cavities of
bivalves, the beautiful brood-shell of the argonaut, illustrate a
habit even an outline of which is beyond our limits.
§ 10. Nemesis of Reproduction, — We have already shown
how reproduction in its origin is linked to death. The primi-
tive ruptures by which the protozoon reduces encumbering
bulk, saves its own life, and multiplies its kind, are only a step
or two from more difluse dissolution which is death.
The association of death and reproduction is indeed patent
enough, but the connection is in popular language usually
misstated. Organisms, one hears, have to die; they must
therefore reproduce, else the species would come to an end.
But such emphasis on posterior utilities is almost always only
an afterthought of our invention. The statement must be
corrected by another; as Goette says, **it is not death that
makes reproduction necessary, but reproduction has death as
its inevitable consequence." This of course refers primarily to
the incipient forms of both these katabolic processes.
It is necessary to give a few illustrations. Goette refers to
Haeckel's Magosphara^ a protozoon which just as it had formed
for itself a multicellular body broke up into the component
SI>ECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 273
units. These lived on, and there was no corpse, but at the
same time the multicellular colony was no more. Again he
takes the case of the lowly and somewhat enigmatical ortho-
nectids, which Van Beneden has classed as Mesozoa, between
the single-celled and the stable many-celled animals. In some
of these the mature female forms numerous germ-cells, and
terminates her individual life by bursting. The germs are
A figure of cell division suggesting the internal disruptions and
rearrangements of the nucleus (a) and protoplasm. — From
Rauber.
liberated, the mother animal has been sacrificed in reproduc-
tion. "The death is an altogether inevitable consequence of
the reproduction."
Nor is this sacrifice confined to the incipient multicellular
organisms. Thus in some species of the annelid Polygordiusy
the mature females break up and die in liberatiDg their ova,
x8
iJ4 tHE EVOLUTION OF SEX.
In the " Heteronereis," or sexually modified form of Nereid
worms, the whole animal dies after the emission of tha genital
products. This is approached, but suggestively avoided, in
some otiier Polychaet worms, e.g., the Syllida: and the CZ/A*-
niasius. The whole organism is not sacrificed, but only a
modified portion of the body. This is probably also the case
with ihe famous Palolo-worm {Eunice viridis). Here in fact
we have one of the keynotes to reproductive differentiation, —
the sacrifice is lessened, and the fatality thus warded ofE
erjulb
But again, we find in some threadworms or nematodes
{eg., Ascaris dactyturis) that the young live at the expense of
the mother, until she is reduced to a mere husk. In fresh-
water Poly^oa, Kraepelin notes that the ciliated embryo leaves
the maternal body-cavity through a pro!apius uteri of the sacri-
ficed mother. In the precocious reproduction of some midge
larva {Chironomus, &c.), the production of young Is fatal
through successive generations.
SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 27$
Both Weismann and Goette, though with different interpre-
tations, note how many insects (locusts, butterflies, ephemerids,
&c.) die a few hours after the production of ova. The ex-
haustion is fatal, and the males are also involved In fact, as
we should expect from the katabolic temperament, it is the
males which are especially liable to exhaustion. The males of
some spiders normally die after impregnating the female, a fact
perhaps helping to throw light upon the sacrifice of others to
their mates. The similarly tiny (ultra-katabolic) male rotifer —
an ideal but too unpractical lover, with not even an alimentary
canal — would seem usually to fail and expire prematurely,
leaving the female to undisturbed parthenogenesis. Every one
is familiar with the close association of love and death in the
common mayflies. Emergence into winged liberty, the love-
dance and the process of fertilisation, the deposition of eggs
and the death of both parents, are often the crowded events
of a few hours. In higher animals, the fatality of the repro-
ductive sacriflce has been greatly lessened, yet death may
tragically persist, even in human life, as the direct nemesis of.
love.
The temporarily exhausting effect of even moderate sexual
indulgence is well known, as well as the increased liability to
all forms of disease while the individual energies are thus
lowered.
§ II. Organic Immortality, — Comparatively little is yet
known about the length of life among lower animals, but there
is no reason to doubt that all multicellular organisms die. We
have just emphasised the view of Goette, and other naturalists,
that reproduction is the beginning of death; which is not
inconsistent with the apparent paradox, that local death was
the beginning of reproduction. Allowing, then, that multi-
cellular organisms at any rate are mortal, and that the very
blossoming of the life in reproduction is fated with a prophecy
of death which is its own fulfilment, we have to face two
questions, — What of death in the Protozoa? and, In what
sense is there an immortality throughout the organic series ?
Often enough already, in the preceding pages, we have had
to reiterate the contrasts between the Protozoa and the higher
animals. These firstlings are single cells physiologically com-
plete in themselves, and have at least very great, if not un-
limited, powers of self-recuperation. They leave off where
higher animal life begins, that is to say, in a unicellular state.
276 THE EVOLUTION OF SEX.
They do not form " bodies." Their reproduction, moreover, is
in the majority simple cell-division into two. If there be loss
of individuality, there is hardly loss of life. Death is not so
serious when there is nothing left to bury. Nor in most cases
can one half of the divided unit be the mother individual, and
the other the daughter, for the two appear indistinguishably
the same. Thus an idea, broached long ago by Ehrenberg,
has been revived and elaborated by several naturalists, and
especially by Weismann, that the Protozoa are virtually im-
mortal.
In VVeismann's own words, " Natural death occurs only
among multicellular organisms, the single-celled forms escape
it There is no end to their development which can be
likened to death, nor is the rise of new individuals associated
with the death of the old. In the division the two portions
are equal, neither is the older nor the younger. Thus there
arises an unending series of individuals, each as old as the
species itself, each with the power of living on indefinitely,
ever dividing but never dying." Ray Lankester puts the
matter tersely, "It results from the constitution of the proto-
zoon body as a single cell, and its method of multiplication by
fission, that death has no place as a natural recurrent pheno-
menon among these organisms."
Some limitations must be noticed, which make this idea of
pristine immortality yet more emphatic. It is only asserted
that the Protozoa escape "natural death," a violent fate may
of course await them like any other organisms. They have no
charmed life, being as liable to be devoured as those of higher
degree. In relation to the environment, however, their sim-
plicity gives them a peculiar power of avoiding impending
destiny. The habit of forming protective cysts is very general,
and thus enwrapped they can, like the ova and a few of the
adults of some higher animals, endure even prolonged desic-
cation with successful patience, which is rewarded by a re-
juvenescence when the rain revisits the pools. But the
doctrine of the "immortality of the Protozoa" refers to a
defiance of natural, not violent, death.
The psychological objection that the original individuality
is extinguished when it divides into two, intrudes a con-
ception which is hardly applicable. The individualities are
doubled, nothing is really lost Most seriously difficult are
those cases where the protozoon produces a series of buds,
SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 277
spores, or division units, and leaves a residual core or unused
remnant behind to die. But in regard to the gregarines, for
instance, where such a remnant is often left, it has been fairly
answered that the residue is rather a kind of excretion than
the parent left to perish after its reproductive sacrifice. Weis-
mann is, however, willing to admit the possibility, that in the
suctorial Acinetae, and in the parasitic gregarines, which are
both somewhat removed from the normal protozoon type,
there may be cases of true mortality.
Another point in regard to which experts differ, is whether
the Protozoa are really quite self-recuperative. They suffer
injuries, they necessarily suffer waste, portions are used up
and ejected. The question then arises. Are those acquired
defects obliterated, or do they become intensified? Is the
wasting only a local death, or is it the beginning of a true
senescence ? This is a question which can only be answered
by observation ; a priori reasoning is here futile. The most
serious criticism of Weismann's view is due to Maupas.
Already we have noted his important result, that conjugation
is essential to the health of the species. Without this incipient
sexual reproduction, the individuals in the course of numerous
successive asexual generations grow old. The nucleus degen-
erates, the size diminishes, the entire energy wanes, the senility
ends in death. Maupas believes that all organisms are fated
to suffer decay and death, and protests strongly against Weis-
mann's theory that death began with the Metazoa.
It must be noted, however, that in natural conditions the
conjugation, prohibited in Maupas's experiments, occurs when
it is wanted, and the life flows on. Furthermore, in many
Protozoa conjugation has not been shown to occur. It seems
therefore more warrantable to insert Maupas's result as a saving
clause to Weismann's doctrine, than to regard it as contra-
dictory. The conclusion at present justifiable, is that Protozoa
not too highly differentiated, living in natural conditions
where conjugation is possible, have a freedom from natural
death. To this must then be added the demonstrated saving
clause, that in ciliated infusorians, conjugation, which here
means an exchange of nuclear elements, is the necessary con-
dition of eternal youth and immortality.
Accepting then, with an emphasised proviso, the general
conclusion that most, if not all, unicellular organisms enjoy
immortality, that in being without the bondage of a " body "
7jS THE EVOLUTION OF SEX.
they are necessarily freed from death, we pass to consider the
second question, What does the death of the higher and multi-
cellular organisms really involve?
If death do not naturally occur in the Protozoa, it is evident
that it cannot be an inherent characteristic of living matter.
Yet it is universal among the multicellular animals. Death,
we may thus say, is the price paid for a body, the penalty its
attainment and possession sooner or later involves. Now, by
a body is meant a complex colony of cells, in which there is
more or less division of labour, where the component units are
no longer, like the Protozoa, in |X)ssession of all their faculties,
but through division of labour have only restricted functions
and limited powers of self-recuperation. Like Maupas's isolated
family of infusorians, the cells of the body do not conjugate
with one another; and though they divide and re-divide for a
season, the life eventually runs itself out.
The relation between reproductive cells and the body. The horizontal ch.iin of cells
represents a succession of the ova from which the "bodies" are produced. At
each generation, a spermatoxoon fertilising the liberated ovum is also indicated.
A moment's consideration, however, will show that in most
cases the organism does not wholly die. Some of the cells
usually escape from the bondage of the body as reproductive
elements, — as, in fact. Protozoa once more. The majority of
these may indeed be lost; eggs which do not meet with male
elements perish, and the latter have even less power of inde-
pendent vitality. But when the ova are fertilised, and proceed
to develop into other individuals, it is plain that the parent
organisms have not wholly died, since two of their cells have
united to start afresh as new plants or animals. In other
words, what is new in the multicellular organism, namely, the
" body," does indeed die, but the reproductive elements, which
correspond to the Protozoa, live on.
This may be made more definite in the preceding diagram.
There it is seen that the organism starts like a protozoon, as a
single cell, or usually as a union of two cells in the fertilised
SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 279
ovum. This divides, and its daughter-cells divide and re-divide.
They arrange themselves in layers, and are gradually mapped
out into the various tissues or organs. In division of labour,
they become restricted in their functions, and specialised in
their structure. They become differentiated as muscle-cells,
nerve-cells, gland-cells, and so on. The result is a more or
less complex " body," unstable in its equilibrium because of its
very complexity, composed moreover of competing cells far
removed from the protozoan all-roundness of function, limited
in their powers of recuperation, and emphatically liable to local
and periodic, or to general and final death. But the body is
not all. At an early stage in some cases, sooner or later
always, reproductive cells are set apart. These remain simple
and undiflferentiated, preserving the structural and functional
traditions of the original germ-cell. These cells, and the results
of their division, are but little implicated in the differentiation
which makes the multicellular organism what it is; they remain
simple primitive cells like the Protozoa, and in a sense they
too share the protozoon immortality. The diagram shows how
one of these cells, separated from the parent organism (and
uniting in most cases with a germ-cell of different origin),
becomes the beginning of a new body, and, at the same time,
necessarily the origin of a new chain, or rather of a continued
chain of fresh reproductive cells.
" The body or soma^^ Weismann says, " thus appears to a
Certain extent as a subsidiary appendage of the true bearers of
the life, — the reproductive cells." Ray Lankester has again
well expressed this: — " Among the multicellular animals, certain
cells are separated from the rest of the constituent units of the
body, as egg-cells and sperm-cells; these conjugate and continue
to live, whilst the remaining cells, the mere carriers as it were
of the immortal reproductive cells, die and disintegrate. The
bodies of the higher animals which die, may from this point of
view be regarded as something temporary and non-essential,
destined merely to carry for a time, to nurse, and to nourish
the more important and deathless fission-products of the
unicellular egg."
In most cases, as Weismann insists, it is more correct to
speak of " the continuity of the germinal protoplasm " than of
the continuity of the germ-cells; but, with this proviso, the
diagram expresses a fact most important in understanding
reproduction and heredity, that the chain of life is in a real
28o THE EVOLUTION OF SEX.
sense continuous, and that the "bodies" which die are
deciduous growths, which arise round about the real links.
The bodies are but the torches which burn out, while the living
flame has passed throughout the organic series unextinguished.
The bodies are the leaves which fall in dying from the con-
tinuously growing branch. Thus although death take inexor-
able grasp of the individual, the continuance of the life is still
in a deep sense unaffected; the reproductive elements have
already claimed their protozoan immortality, are already
recreating a new body; so in the simplest physical, as in the
highest psychic life, we may say that love is stronger than
death.
SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 28 1
SUMMARY.
I. According to Weismann's theory of the continuity of the germ-plasm,
a portion of the specific hereditary substance which the fertilised ovum
contains is not used up in the development of the offspring's body, but is
reserved unchanged to form the germ-cells of the following generation.
2. Sexual maturity generally occurs towards the limit of growth, is marked
by liberation of reproductive elements and by secondary characteristics, in
part due to the reaction of the reproductive function on the general system.
Precocious maturity may be due to constitutional or environmental con-
ditions, and has been of much importance in the evolution of flowering
plants.
3. Menstruation may be interpreted as a means of getting rid of the
anabolic surplus of the female in absence of its foetal consumption.
4. Sexual union, at first very passive and random, becomes active and
definite with the gradual evolution of sex and secondary sexual organs.
5. Birth is at first accomplished by rupture, but becomes a definite
process usually effected through special ducts. Oviparous and viviparous
birth only differ in degree.
6. Early nutrition is usually an absorption of the yolk, but in mammals
is accomplished by osmotic transfusion from the blood of the mother to that
of the foetus.
7. Lactation may be interpreted as an anabolic overflow.
8. Besides milk, there are other secretions associated with the nutrition
and sheltering of the young. Pigeon's milk, edible birds' nests, and the
mucous threads of sticklebacks, are illustrations.
9. Incubation, reaching a climax in birds, is paralleled in many other
classes.
10. Reproduction and death both represent katabolic crises. Primitively,
they are nearly akin. Reproduction may ward off death from the Proto-
zoon, but in the simplest Metazoa it helped to cause it.
I I. The Protozoa come nearer immortality than other organisms. The
fact of germinal continuity involves an organic immortality.
LITERATURE.
For the special physiology of sex and reproduction, consult standard text-
books, such as those of Foster, Landois and Stirling, and especially Hensen's
work already often cited.
On the continuity of the germ-plasma, consult recent translation of
Weismann's papers — " Heredity," Oxford, 1889; while a full bibliography
will be found in " History and Theory of Heredity," by J. A. Thomson,
Proc. Roy. Soc. Edin., 18S8; and, since 1886, in the Zoological Record.
On the nemesis of reproduction, and on organic immortality, see A.
Goette, **Ueber den Ursprung des Todes," Hamburg and Leipzig, 1883;
and A. Weismann, ** Ueber die Dauer des Lebens," Jena, 1882; ** Ueber
Leben und Tod," Jena, 1S84; E. Maupas, "Archives de Zoologie
experimental," 1888.
282 THE EVOLUTION OF SEX.
Beard, J. — The Span of Gestation and the Cause of Birth: a Study of the
Critical Period and its Effects in Mammalia. Jena, 1897, ix. and
13^ pp.
^he]
The Rhythm of Reproduction in Mammals. Anat. Anzeiger, XIV.,
1897, pp. 97.102.
Bebton, Alice, and Pearson, K. — Inheritance of Longevity. Proc.
Roy. Soc. London, 1899; Nature, LX., 1899, PP* 356-357'
Ellis, Havelock. — Psychology of Sex, Vol. II., 1899. (In great part
devoted to a discussion of pathological sexual conditions.) Man and
Woman. London, 1894.
Fer£, Ch. — Les Perversions Sexuelles chez les Animaux. Revue Philo-
sophique, XXII., 1897, pp. 494-503.
Heape, W. — The Artificial Insemination of Mammals. Proc. Roy. Soc
London, LXI., 1897, pp. 52-63.
KoHLWBY, H.— Arten- und Rassenbildung. Eine EinfUhrung in das
Gebiet der Tierzucht. Leipzig, 1897, 72 pp., 5 figs.
Lee, F. S. — Physiology of Reproduction in W. H. Howell's Text-book of
Physiology. Philadelphia, 1896.
Mackenzie, J. N. — Ph3^ological and Pathological Relations between the
Nose and the Sexual Apparatus of Man. Bulletin Johns Hopkins
Hospital, IX., 1898, pp. 10-17.
Morgan, C. Lloyd. — Habit and Instinct. London, 1896, pp. 351.
(See chapter on habits of birds, etc., in mating.)
Pearson, Karl. — Law of Ancestral Heredity. Proc Roy. Soc London,
LXI I., 1898, pp. 386-412. The Chances of Death, and other Studies
in Evolution, 2 vols. London, 1897.
Webster, J. C— tlie Biological Basis of Menstruation. Montreal Med.
Journal, April 1897, 19 pp.
CHAPTER XIX.
Psychological and Ethical Aspects.
§ I. Common Ground between Animals and Men, — Hitherto
we have been justifying the orthodoxy of an anatomical train-
ingf by almost wholly ignoring the fact that animals have a
psychic life, or only mentioning the mere neural aspect of
functions. Only in discussing sexual selection, and the general
facts of sexual union and of parentage, have we intruded words
like **care," "sacrifice," and "love." A purely physiological
treatment of sex and reproduction is, however, obviously
incomplete. It would be rejected with scorn in reference to
human life; it must be equally rejected in regard to the
higher animals, which, taken together, exhibit the analogues
of almost every human emotion, and of all our less recondite
intellectual processes. It is with emotions that we have here
most to do ; and without raising the difficult question whether
animals exhibit any emotions exactly analogous to those which
in man are associated with the " moral sense," " religion," and
" the sublime," we accept the conclusion of Darwin, followed
by Romanes and others, that all other emotions which we
ourselves experience, are likewise recognisable in analogous
expression in the higher animals. Those which are associated
with sex and reproduction are indeed among the most patent ;
love of mates, love of offspring, lust, jealousy, family affection,
social sympathies, are undeniable.
§ 2. The Love of Mates, — In the lowest animals, where
two exhausted cells flow together in incipient sexual union,
there is apparently only one component of that most complex
musical chord in life which we call " love." There is physical
attraction, and the whole process is very much a satisfaction of
protoplasmic hunger.
In multicellular animals, the liberation of sex-elements is
at first very passive. It concerns the individual alone. Fertili-
sation is a random matter; and though sex exists, sexual
attraction does not«
284 THE EVOLUTION OF SEX.
A grade higher, true sexual union begins to appear. But
at first this simply occurs between any male and any available
female. The psychological factor is still but feebly expressed;
there is no genuine pairing, and it would be folly to use the
word love in such cases.
Gradually, however, for instance among insects, the sexes
associate in pairs. There is some psychic sexual attraction,
often accompanied with no little courtship, but much more
important is the occasional maintenance of the association for
a lengthened period. There may even be co-operation in
work, as in dung>rolling beetles such as Ateuchus^ where the
two sexes pursue their somewhat disinterested labours together.
The male and female of another lamellicorn beetle (Ze/hrus
cephalotes) inhabit the same cavity, and the virtuous matron
is said greatly to resent the intrusion of another male. As
degenerate offshoots from the path of psychic progress, or as
illustrations of the predominance of merely physical attraction,
one must regard such prolonged associations of the two sexes
as are seen in the formidable parasitic worm Btiharzia^ where
the male carries the female about, or in some parasitic crusta-
ceans where the positions are reversed.
Among the cold-blooded fishes, the battles of the stickle-
back with his rivals, his captivating manoeuvres to lead the
female to the nest which he has built, his mad dance of
passion around her, and his subsequent jealous guarding of
the nest, have often been observed and admired. In one of
the sunfishes the male and female alternate in guarding the
ova. The monogamous habits of the salmon, and the
frequently fatal contests between rival males are well known.
Carbonnier has beautifully described the elaborateness of
sexual display and the ardency of passion in the male butterfly-
fish, and also in the rainbow-fish of the Ganges.
The amatory croaking of frogs, the love-gambols of some
newts, the curious parental care of some male amphibians
mentioned in the preceding chapter, and the like, illustrate
the continuance of more than crude physical attraction
between the sexes. Of many amphibians it may be said that
it is only in their sexual and reproductive relations that they
seem to wake up out of their constitutional sluggishness.
In regard to reptiles, little is known beyond the exhibition
of sexual passion and the jealous combats of rival males. Yet
Romanes refers to the interesting fact that when a cobra is
PSYCHOLOGICAL AND ETHICAL ASPECTS. 28$
killed, its mate is often found on the same spot a day or two
afterwards.
Among birds and mammals, the greater diflferentiation of
the nervous system and the higher pitch of the whole life is
associated with the development of what pedantry alone can
refuse to call love. There is often partnership, co-operation,
and evident affection beyond the limits of the breeding periods,
there are abundant illustrations of regard for conventions,
there are close analogues of human flirtation, courtship,
jealousy, and even crime, though, so far as we understand the
matter, there is no convincing evidence of what may be called
a distinctively moral judgment. There is no doubt that in
the two highest classes of animals at least, the physical
sympathies of sexuality have been enhanced by the emotional,
if not also intellectual, sympathies of love. .Those sceptical
on this point should consult such a work as Biichner's ** Liebe
und Liebesleben in der Thierwelt," or Sutherland's "Origin
and Growth of the Moral Instinct" (1898), which contain an
overflowing wealth of instances.
§ 3. Sexual Attraction. — Mantegazza has written a work
entitled " The Physiology of I-.ove," in which he expounds
the optimistic doctrine that love is the universal dynamic;
and from this Biichner quotes the sentence, that "the whole
of nature is one hymn of love." If the last word be used very
widely, this often-repeated utterance has more than poetic
significance. But even in the most literal sense there is much
truth in it, since so many animals are at one in the common
habit of serenading their mates. The chirping of insects, the
croaking of frogs, the calls of mammals, the song of birds,
illustrate both the bathos and glory of the love-chorus. The
works of Darwin and others have made us familiar with the
numerous ways, both gentle and violent, in which mammals
woo one another. The display of decorations in which many
male birds indulge, the amatory dances of others, the love-
lights of glow-insects, the joyous tournaments or furious duels
of rival suitors, the choice which not a few females seem to
exhibit, and the like, show how a process, at first crude
enough, becomes enhanced by appeals to more than merely
sexual appetite. But it is hardly necessary now to argue
seriously in support of the thesis that love — in the sense of
sexual sympathy, psychical as well as physical — exists among
animals in many degrees of evolution. Our comparative
286 THE EVOLUTION OP SEX.
psychology has been too much influenced by our intellectual
superiority ; but while this, no doubt, has its correspondingly
increased possibilities of emotional range, it does not neces-
sarily imply a corresponding emotional intensity ; and we
have no means of measuring, much less limiting, that glow of
organic emotion which so manifestly flushes the organism with
colour and floods the world with song. Who knows whether
the song-bird be not beside the man what the child-musician
is to the ordinary dulness of our daily toil and thought ? The
fact to be insisted upon is this, that the vague sexual attraction
of the lowest organisms has been evolved into a deflnite repro-
ductive impulse, into a desire often predominating over even
that of self-preservation; that this again, enhanced by more
and more subtle psychical additions, passes by a gentle
gradient into the love of the highest animals, and of the
average human individual.
But the possibilities of evolution are not ended, and
though some may shrink from that comparison of human
love with its analogues in the organic series, the theory of
evolution oflers the precise compensation such natures require.
Without recognising the possibilities of individual and of racial
evolution, we are shut up to the conventional view that the
poet and his heroine alike are exceptional creations, hopelessly
beyond the everyday average of the race. Whereas, admitting
the idea of evolution, we are not only entitled to the hope, but
logically compelled to the assurance, that these rare fruits of
an apparently more than earthly paradise of love, which only
the forerunners of the race have been privileged to gather, or
it may be to see from distant heights, are yet the realities of a
daily life towards which we and ours may journey.
§ 4. Intellectual and Emotional Differences between the
Sexes, — We have seen that a deep difference in constitution
expresses itself in the distinctions between male and female,
whether these be physical or mental. The differences may be
exaggerated or lessened, but to obliterate them it would be
necessary to have all the evolution over again on a new basis.
What was decided among the prehistoric Protozoa cannot be
annulled by Act of Parliament. In this mere outline we
cannot of course do more than indicate the relation of the
biological difflerences between the sexes to the resulting
psychological and social differentiations; for more than this
neither space nor powers suffice. We must insist upon the
PSYCHOLOGICAL AND ETHICAL ASPECTS. 287
biological considerations underlying the relation of the sexes,
which have been too much discussed by contemporary writers
of all schools as if the known facts of sex did not exist at all,
or almost if these were a mere matter of muscular strength or
weight of brain.
The reader need not be reminded of the oldest and most
traditional views of the subjection of women inherited from the
ancient European order; still less perhaps of the attitude of
the ordinary politician, who supposes that the matter is one
essentially to be settled by the giving or withholding of the
franchise. The exclusively political view of the problem has
in turn been to a large extent subordinated to that of economic
iaisseZ'faire, from which of course it consistently appeared that
all things would be settled as soon as women were sufficiently
plunged into the competitive industrial struggle for their own
daily bread. While, as the complexly ruinous results of this
inter-sexual competition for subsistence upon both sexes and
upon family life have begun to become manifest, the more
recent economic panacea of redistribution of wealth has
naturally been invoked, and we have merely somehow to raise
women's wages.
All disputants have tolerably agreed in neglecting the
historic, and still more the biological factors ; while, so far as
the past evolution of the present state of things is taken into
account at all, the position of women is regarded as having
simply been that in which the stronger muscle and brain of
man was able to place her. The past of the race is thus
depicted in the most sinister colours, and the whole view is
supposed to be confirmed by appeal to the practice of the
most degenerate races, and this again as described with the
scanty sympathy or impartiality of the average white traveller,
missionary, or settler.
As we have already said, we cannot attempt a full discus-
sion of the question, but our book would be left without
point, and its essential thesis useless, if we did not, in con-
clusion, seek to call attention to the fundamental facts of
organic difference, say rather divergent lines of differentiation,
underlying the whole problem of the sexes. We shall only
suggest, as the best argument for the adoption of our stand-
point, the way in which it becomes possible relatively to
harmonise the very diverse outlooks. We shall not so readily
abuse the poor savage, who lies idle in the sun for days after
288 THE EVQLUTION OF SEX.
his return from the hunting, while his heavy-laden wife toils
and moils without complaint or cease ; but bearing in view the
extreme bursts of exertion which such a life of incessant
struggle with nature and his fellows for food and for life in-
volves upon him, and the consequent necessity of correspond-
ingly utilising every opportunity of repose to recruit and eke
out the short and precarious life so indispensable to wife and
weans, we shall see that this crude domestic economy is the
best, the most moral, and the most kindly attainable under the
circumstances. Again, the traveller from town, who thinks
the agricultural labourer a greedy brute for eating the morsel
of bacon and leaving his wife and children only the bread,
does not see that by acting otherwise the total ration would
soon be still further lowered, by diminished earnings, loss of
employment, or loss of health.
The actual relations of fisherman and fishwife, of the
smallest farmer and his wife, seem to us to give a truer as well
as a healthier picture of antique industrial society, than those
we find in current literature ; and if we admit that such life
is deficient in refinement (although, on all deeper grounds,
from religion to ballad poetry, we might even largely dispute
this), it has still much to teach in respect of simplicity and
health.
The old view of the subjection of women was not, in fact,
so much of tyranny as it seemed, but roughly tended to
express the average division of labour; of course hardships
were frequent, but these have been exaggerated. The abso-
lute ratification of this by law and religion was merely of a
piece with the whole order of belief and practice, in which
men crushed themselves still more than their mates. Being
absolute, however, such theories had to be overthrown, and
the application of the idea of equality, which had done such
good service in demolishing the established castes, was a natural
and serviceable one. We have above traced the development
of this, however, and it is now full time to re-emphasise, this
time of course with all scientific relativity instead of a dogmatic
authority, the biological factors of the case, and to suggest
their possible service in destroying the economic fallacies at
present so prevalent, and still more towards reconstituting that
complex and sympathetic co-operation between the differen-
tiated sexes in and around which all progress past or future
must depend. Instead of men and women merely labouring
PSYCHOLOGICAL AND ETHICAL ASPECTS, 289
to produce things as the past economic theories insisted, or
competing over the distribution of them, as we at present
think so important, a further swing of economic theory will
lead us round upon a higher spiral to the direct organic facts.
So it is not for the sake of production or distribution, of self-
interest or mechanism, or any other idol of the economists,
that the male organism organises the climax of his life's
struggle and labour, but for his mate ; as she, and then he,
also for their little ones. Production is for consumption ; the
species is its own highest, its sole essential product. The
social order will clear itself, as it comes more in touch with
biology.
It is equally certain that the two sexes are complementary
and mutually dependent. Virtually asexual organisms, like
Bacteria, occupy no high place in Nature's roll of honour ;
virtually unisexual organisms, like many rotifers, are great
rarities. Parthenogenesis may be an organic ideal, but it is
one which has been rarely realised. Males and females, like
the sex-elements, are mutually dependent, and that not merely
because they are males and females, but also in functions not
directly associated with those of sex. To dispute whether
males or females are the higher, is like disputing the relative
superiority of animals and plants. Each is higher in its own
way, and the two are complementary.
While there are broad general distinctions between the in-
tellectual, and especially the emotional, characteristics of males
and females among the higher animals, these not unfrequently
tend to become mingled. There is, however, no evidence that
they might be gradually obliterated. The males of the sea-
horse, the obstetric frog, and many birds discharge maternal
functions, and there are females who fight for the males, and
are stronger, or more passionate than their mates. But these
are rarities. It is generally true that the males are more active,
energetic, eager, passionate, and variable ; the females more
passive, conservative, sluggish, and stable. The males, or, to
return to the terms of our thesis, the more katabolic organisms,
often seem more variable, and therefore, as Brooks has empha-
sised, may have frequently been the leaders in evolutionary
progress, while the more anabolic females tend rather to pre-
serve the constancy and integrity of the species.
There are some cases, as illustrated notably by the contrast
between ruffs and reeves, where the greater variability of the
'9
290 THE EVOLUTION OF SEX.
males along certain lines seems obvious, but, according to Karl
Pearson, tlie doctrine that man is more variable than woman
is a pseudo scientific superstition, based on inadequate or in-
admissible data. The examination of seventeen groups of
measurements of different parts of the body shows that in
eleven groups the female is more variable than the male, and
in six the male more than the female. The differences of
variability, however, are slight, less than those between
members of the same race living in different conditions;
they are perhaps due to differences in the severity of the
struggle for existence. (See "The Chances of Death, and
other Studies in Evolution," 2 vols., London, 1897, pp. 388
and 460.)
Along paths where the reproductive sacrifice was one of
the determinants of progress, the females must have the credit
of leading the way. The. more active males, with a conse-
quently wider range of experience, may have bigger brains and
more intelligence; but the females, especially as mothers, have
indubitably a larger and more habitual share of the altruistic
emotions. The males being usually stronger, have greater
independence and courage ; the females excel in constancy of
affection and in sympathy. The spasmodic bursts of activity
characteristic of males contrast with the continuous patience
of the females, which we take to be an expression of con-
stitutional contrast, and by no means, as some would have us
believe, a mere product of masculine bullying. The stronger
lust and passion of males is likewise the obverse of pre-
dominant katabolism.
That men should have greater cerebral variability and
therefore more originality, while women have greater stability
and therefore more *' common sense," are facts both con-
sistent with the general theory of sex and verifiable in common
experience. The woman, conserving the effects of past varia-
tions, ha? what may be called the greater integrating intel-
ligence; the man, introducing new variations, is stronger in
differentiation. The feminine passivity is expressed in greater
patience, more open-mindedness, greater appreciation of subtle
details, and consequently what we call more rapid intuition.
The masculine activity lends a greater power of maximum
effort, of scientific insight, or cerebral experiment with im-
pressions, and is associated with an unobservant or impatient
disregard of minute details, but with a stronger grasp of
PSYCHOLOGICAL AND ETHICAL ASPECTS. 29I
generalities. Man thinks more, women feels more. He dis-
covers more, but remembers less ; she is more receptive, and
less forgetful.
§ 5. Tlie Love /or Offspring, — Just as it is impossible to
point to the stage where psychical sympathies enhance the re-
productive impulse into the love of mates, so we cannot tell
where parental care becomes disinterested enough to warrant
our calling it love of offspring. For, as no one can be foolish
enough deliberately to ignore the sexual or physical basis of
"love" in the higher and highest organisms, so it must be
allowed that even maternal care has its selfish side. To take
only one example, that of lactation. The unrelieved pressure
in the mammary glands of a mother animal robbed of her
young is no doubt largely concerned in prompting her to
adopt young ones not her own, yet we soon see these estab-
lished in her affections. So in normal cases, there naturally
remains an alloy which prevents us from regarding even ma-
ternal care as altogether disinterested. In all such cases, our
interpretations risk an undue materialism on the one hand,
and an undue transcendentalism on the other ; and while our
modern temper may habitually incline us to the former, we
must not be too fond of taking for granted that all the
common-sense is on that side, for we must remember that the
course of evolution not only has been, but must be, towards
the other.
Among animals low down in the organic series there is
often a close association between mother and offspring. Even
in some coelenterates and worms the offspring cling about the
mother animals, and may be protected in various kinds of
brood-chambers. The little freshwater leech, Clepsine^ carries
its young about with it, fixed to its ventral surface. A marine
leech, known as the skate-sucker {Poniobdella muricata\
mounts guard for weeks over the eggs which are laid in a
bivalve shell or the like. It is probable that this habit has
protective value, but whether from active enemies or from
accumulations of sand and mud is uncertain. In an aquarium
one of these leeches continued to incubate for one hundred
and twenty-three days.
In some sea-urchins and starfishes there are simple forms
of brood-care, but the case of Holothurians is perhaps more
interesting. Prolonged attachment between the young ones
and the mother is known in at least nine species, of which five
I9> THE EVOLUTIOS OF SEX.
are antarctic and one arctic. The mode of attachment difTers
markedly in different forms, thus each of the five antarctic
MYCHOLOGICAL AND EtHICAL ASPECTS. 293
Species has its young attached in a dtfTerent way. In Fsolus
ephippifer the young develop among the dorsal plates; in Psoius
anlarclictts, on the ventral surface; in Cvcumariacrocea, on the
modified dorsal ambulacra; in Cuciimaria leevigala, in ventral
pouches; and in Chirodota contorts, in the genital tubes. (See
H. Ludwig, "ZooL Anzeiger," xx., 1897, pp. 217-219.) The
interpretation here evidently does not lie with the morpho-
Ic^ist; is he not compelled to speculate on the beginnings of
psychic life in the strangely rudimental nervous system of
these forms from which even definite ganglia seem absent?
Yet even here we have motherhood protecting offspring,
perhaps all the more because of the prevailing cold; perhaps
2Q4 THE EVOLUTION OF SEX,
of course also as a protection from premature burial in sort
muddy bottoms.
In some lowly crustaceans, the young may return to the
shell-cavity of the mother after hatching, and even after they
have undergonea moult. The young craylish are said to return
to the maternal shelter after they have been set adrift. The
care of the nurse-bees for their charge, though not exactly
maternal, deserves to be recalled ; and the way in which ants
save the cocoons when danger threatens is well known. De
EgE-Clusleis ofa species of Culllifiah.— From Von Hayuk.
Geer describes how one of the insects infesting plants behaves
to her young brood exactly like a hen with her chickens; and
iJonnet vividly describes a case where n mother spider, at the
mercy of an ant-lion, fought for her eggs at the sacrifice of her
own life. Some spiders, too, carry their young; and some
crustaceans swim along with their young ones. Some cuttle-
fishes are careful in keeping their egg clusters clean and safe;
while even the headless fresh-water mussel retains her young.
J'SYCHOLOGICAL AND ETHICAL ASPECTS.
^9i
when there is no fish present to which tbey may attach them-
selves. In fishes, it must be allowed that the care, if at all
evident, is usually paternal; in amphibians, it is rare; in
Ideal unity.
N
society.
•1— 1
.
A
r~-i
bC 1
>^5^^
^..•■•^y fl
f c:
\ \
^^^
^^^ 1
cr family.
I
\^^
"^vf
1
-, H
Iv
'Cf
1
w ^
1
4)
\^
if offspring.
o
OB "^
IL
J§
' ^ "
^.
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/A
^
mates.
N V R
Protoplasmic identity.
Diagrammatic Representation of the Relations between Nutritive,
Seir-Maintaining, or Egoistic, and Reproductive, Species-
Regarding, or Altruistic Activities.
reptiles, somewhat more marked. In birds and mammals,
however, parental care is general, and unquestionably grows
into love for offspring.
§ 6. Egoism and Altruism. — The optimism which finds in
296 The Evolution of sex.
animal life only " one hymn of love " is inaccurate, like the
pessimism which sees throughout nothing but selfishness.
Littrd, Leconte, and some others less definitely, have more
reasonably recognised the co existence of twin streams of
egoism and altruism, which often merge for a space without
losing their distinctness, and are traceable to a common origin
in the simplest forms of life. In the hunger and reproductive
attractions of the lowest organisms, the self-regarding and
other-regarding activities of the higher find their starting-point
Though some vague consciousness is perhaps co-existent with
life itself, we can only speak with confidence of psychical
egoism and altruism after a central nervous system has been
definitely established. At the same time, the activities of even
the lowest organisms are often distinctly referable to either
category.
A simple organism, which merely feeds and grows, and
liberates superfluous portions of its substance to start new exist-
ences, is plainly living an egoistic and individualistic life. But
whenever we find the occurrence of close association with
another form, we find the first rude hints of love. It may still
be almost wholly an organic hunger which prompts the union,
but it is the beginning of life not wholly individualistic. Hardly
distinguishable at the outset, the primitive hunger and love
become the starting-points of divergent lines of egoistic and
altruistic emotion and activity.
The differentiation of separate sexes; the production of
offspring which remain associated with the parents; the
occurrence of genuine pairing beyond the limits of the sexual
period; the establishment of distinct families, with unmistak-
able affection between parents, offspring, and relatives; and
lastly, the occurrence of animal societies wider than the
family, — mark important steps in the evolution of both egoism
and altruism.
The diagram sums up the important facts. There are two
divergent lines of emotional and practical activity, — hunger,
self-regarding, egoism, on the one hand ; love, other-regarding,
altruism, on the other. These find a basal unity in the primi-
tively close association between hunger and love, between
nutritive and reproductive needs. Each plane of ascent marks
a widening and ennobling of the activities; but each has its
corresponding bathos, when either side unduly preponderates
over the other. The actual path of progress is represented by
i
PSYCHOLOGICAL AND ETHICAL ASPECTS. 297
action and reaction between the two complementary functions,
the mingling becoming more and more intricate. Sexual
attraction ceases to be wholly selfish; hunger may be over-
come by love; love of mates is enhanced by love for offspring;
love for offspring broadens out into love of kindred. Finally,
the ideal before us is a more harmonious blending of the two
streams.
298 THE EVOLUTION OP SEX.
SUMMARY.
1. In most of the emotions, and in the simpler intellectual processes,
there is common ground between animals and men. This is especially true
of the emotions associated with sex and reproduction.
2. The love of mates has its roots in physical sexual attraction, but has
been gradually enhanced by psychical sympathies.
3. The modes of sexual attraction rise from the crude and physical to
the sul)tle and psychical.
4. The intellectual and emotional differences l>etween the sexes are
correlated with the deep-seated constitutional differences. Males and
females are complementary, each higher in its own way.
5. The love for offspring has grown as gradually as the love for mates.
Even lactation and maternal care may be in part egoistic. Apart from
exceptional cases, genuine love for ofl'spring is only emphatic in birds and
mammals, where the reproductive sacrifice of the mother has also been
increased.
6. Egoism and altruism have their roots in the primary hunger and
love, or nutritive and reproductive activities. The divergent streams of
emotion and activity have a common origin, subtly mingle at various
turning-points^ and ought to blend more and more in one.
LITERATURE.
See works on Sexual Selection cited at Chap. I.
See also Car us Sterne's most admirable of general natural history
books — Werden und Vergehen. Third edition. Berlin, 1886.
BucHNKR, r* — Liebe und Liebesleben in dor Thierwelt. Berlin, 1879.
EiMER, G. H. T. — Die Entstehung dor Arten auf Grund von Vererl»cn
Erworl^ner Eigenschaften nach den Gesetzen Organischen Wachsens.
Jena, 1888.
Ellis, H.— Man and Woman. Contemporary Science Series.
Groos, K. — The Play of Animals. (Translation), 1898.
Mantegazzv, p. — Die Physiologic der Liebe; Die Ilygijne der Liel)c ;
and Anlhropologisch-Kulturhistorische Studien iilxrr die Geschlechts-
verhaltnisse des Menschen. Jena.
Ploss. — Das Weib in der Natur und Vdlkerkunde. Second edition.
Leipzig, 1887.
RoLPH, W. \l.—Op. cit.
Romanes, G. |. — Animal Intelligence. Internal. Sci. Series. Fourth
ediiion, 1886 ; and Mental Evolution in Animals, by the same.
Sutherland, A. — The Origin and Growth of the Moral Instinct. 2 vols.,
1898.
CHAPTER XX.
Laws of Multiplication.
§ I. Rate of Reproduction and Rate of Increase, — We know
much more about the rate at which organisms reproduce, than
about the rate at which the number of adults in reality increases
or decreases. The one fact may be ascertained by observation ;
the other involves comparative statistics, which are difficult
enough to obtain, even for the human species. The rate of
reproduction depends upon the constitution of the individual
and its immediate environment, including, above all, its nutri-
tion. The rate of increase or decrease depends upon the wide
and complex conditions of the entire animate and inanimate
environment, or upon the degree of success in the struggle for
existence.
That there are enormous differences in the rates of repro-
duction is very evident. Maupas tells us how a single infu-
sorian becomes in a week the ancestor of a progeny only
computable in millions, — of numbers which the progeny of a
pair of elephants, supposing they all lived their natural term of
years, would not attain to in ^ve centuries. Again, Huxley
calculates that the progeny of a single parthenogenetic plant-
louse — supposed again to live a charmed life — would in a few
months literally outweigh the population of China. The geo-
metrical ratio of reproduction, so often emphasised, would
indeed have startling results if it involved real, and not merely
potential, increase.
That it does sometimes realise itself for short periods or
special areas of favourable conditions is well known ; for in-
stance, in the periodic plagues of insects, or in the still unmas-
tered rabbit pest of Australia. But in the established fauna
and flora of a country, without intruded importations or marked
climatic changes, the rise and fall of population is seldom
emphatic. The rate of reproduction is only one factor in the
3oO THE EVOLUTION OF SEX.
numerical strength of the species or in its increase. The
common tapeworm produces myriads of embryos, but these
have only one chance in eighty-five millions (it is said) of
succeeding. Many common and numerous animals repro-
duce very slowly. That some species are on the increase, t.g.,
bacteria, under the unprecedentedly favourable conditions which
our recent *'*' industrial progress " affords, while other species
are on the decrease, ^^., many birds, is certain; but the rate
of reproduction is not a direct condition in either case.
§ 2. History of Discussion on RaU of Reproduction, — In this,
as in not a few other cases, the biologist is profoundly indebted
to the student of social questions, for no adequate attention was
paid to the laws of multiplication before the appearance of the
epoch-making " theory of population " of Malthus, nor is it yet
possible or profitable to isolate the human question from the
general one. Malthus's fundamental proposition is indeed
usually softened from its earliest form — that population tends
to increase in geometrical, subsistence only in arithmetical
ratio — into the simple statement that population tends to out-
run subsistence, but has none the less served as a base of
weighty deductions for both the naturalist and the economist
From Darwin's standpoint, the " |X)sitive checks " to population
(disease, starvation, war, infanticide), and the "prudential"
(moral or birth-restricting) checks, come to be viewed as special
forms of natural or artiticial selection, while the fundamental
induction has been extended throughout nature as the essential
condition of the struggle for existence. After long dispute, the
induction of Malthus gained acceptance, followed by wide
deductive use and abuse, among economists. Yet, fundament-
ally important as the subject thus is to naturalist and economist
alike, the former has not as yet effected any thorough investi-
gation of the conditions of multiplication, or even usually
incorporated the keen analysis which we owe to Spencer, while
the economic theorist or disputant frequently still employs the
doctrine even in its pre-Darwinian form. It is thus doubly
needful to summarise, as briefly as may be, Spencer's elaborate
statement of the laws of multiplication.
§ 3. Summary o/Speiuet^s Analysis. — Different species exhibit different
degrees of fertility, which have become established in process of evolution
like the organisms themselves. To understand this particular adaptation
of function to conditions of existence, of organism to environment, we may
analyse these into their respective factors. It is evident that in the environ-
ment of any species there are many conditions with which its individuals
LAWS OF MULTIPLICATION. 3OI
establish a moving equilibrium, sooner or later overthrown in death. To
prevent extinction, the organism meets these environing actions in two
distinct ways, — (i) by individual adaptations, active thrusts or passive
parries ; (2) by the production of new individuals to replace those over-
thrown, — in other words, by genesis. The latter may occur, as we have
seen, in varied forms, sexual or asexual, and at various rates, which depend
upon age, frequency, fertility, and duration of reproduction, together with
amount and nature of parental aid. These actions and reactions of environ-
ment and organism admit of another grouping in more familiar terms, into
two conflicting sets, — (a) the forces destructive of race ; [b) the forces pre-
servative of race.
Leaving aside cases in which permanent predominance of destructive
forces causes extinction, and also, as infinitely improbable, cases of perfectly
stationary numbers, the inquiry is : — In races that continue to exist, what
laws of numerical variation result from these variable conflicting forces
that are respectively destructive or preservative of race ? How is the
alternate excess of one or other rectified? A self-sustaining balance must
exist ; the alternate predominance of each force must initiate a compensa-
tory excess of the other ; how is this to be explained ?
AMien favourable circumstances cause any species to become unusually
numerous, an immediate increase of destructive influences, passive as well
as active, takes place ; competition becomes keener and enemies more
abundant, and conversely. Yet this is not the sole, much less the perma-
nent, means of establishing a balance ; nor does it explain either the
differences in the rate of fertility and mortality, or the adaptation of one to
the other. This minor adjustment in fact implies a major one.
The forces preservative of race were seen above to be two, — power to
maintain individual life, and power to generate the species. Now, in a
species which survives, given the forces destructive of race as a constant
quantity, those preservative of race must be a constant quantity also ; and,
since the latter are two, the individual plus the reproouctive, these must
vary inversely, one must decrease as the other increases. To this law
every species must conform, or cease to exist. Let us restate this at greater
len^h. A species in which self-preservative life is low, and in which the
individuals are accordingly rapidly overthrown in the struggle with the
destructive forces, must become extinct, unless the other race-preservative
factor be proportionally strengthened, — unless, that is to say, its reproductive
power become proportionally great. On the other hand, if both preserva-
tive factors be increased, if a species of high self-preser\'ative power were
also endowed with powers of multiplication beyond what is needful, such
success of fertility, if extreme, woula cause sudden extinction of the species
by starvation, and if less extreme, and so effecting a permanent increase of
the numbers of the species, would next bring about such intenser competi-
tion, such increased dangers to individual \ut, that the great self-preserva-
tive power would not be more than sufficient to cope with them.
In short, then, we have reached the a priori principle, that in races
which continuously survive, in which the destructive forces are balanced
by the preservative ones, there must be an inverse proportion between the
power to sustain individual life and the power to produce new individuals.
But what is the physiological explanation of this adjustment, and how has
it arisen in process of evolutifm ? Spencer has elsewhere enlarged upon the
proposition, which we have already illustrated, that genesis in all its forms
302 THE EVOLUTION OF SEX.
is a process of disintegration, and is thus essentially opposed to that process
of integration which is one element of individual evolution. The matter
and energy supplied for the young organism represent so much loss for
the parent ; while, conversely, the larger the amount of matter and cnerg>'
consumed by the functional actions of the parent, thd less must be the
amount remaining for those of the offspring. The disintegration which
constitutes genesis may l)c complete or partial, and in the latter case the
parent, having reached considerable bulk and complexity before reproduc-
tion sets in, may survive the process. In the same way, individual evolution
may be expressed in bulk, in structure, in amount or variety of action, or in
combinations of these ; yet, in any case, this progress of each individuality
must correspondingly retard the establishment of the new ones.
While in the first portion of the argument, then, it was shown that a
species cannot be maintained unless self- preservative and reproductive
power vary inversely, it is now evident that, irrespective of an end to be
subservedi these powers cannot do other than vary inversely, and the one
a priori principle is thus seen to be the obverse of the other. And if we
group under the term individuation all those race-preservative processes by
whicn individual life is completed and maintained, and extend the term
genesis to include all those processes aiding the formation and perfecting of
new individuals, the result of the whole argument may be tersely expressed
in the formula, — Individuation and Genesis vary inversely. And from this
conception important corollaries open ; thus, other things equal, advancing
evolution must be accompanied by declining fertility ; again, if the difh-
culties of self-preservation permanently diminish, there will be a permanent
increase in the rate of multiplication, and conversely.
In attempting the inductive verification of these a priori inferences,
practical difHculties arise, owing to the high complexity of each of our two
sets of factors and the independent variability of their details, and thus the
total cost of individuation and of genesis alike is hard of estimation and
comparison. For this purpose, however, there are successively to be in-
vestigated, — (i) the antagonism between growth and genesis, sexual and
asexual ; (2) that between development and genesis ; (3) that l^etween ex-
penditure and genesis ; and (4) the coincidence between high nutrition and
genesis. It is impossible to summarise the wealth of evidence drawn from
a wide survey of the animal and vegetable world contained in the chapters
devoted to those various heads, but attention may be called to the last and
most obscure of these. It is indeed evident « /n'^r* that, if the cost of
individuation be once provided for, a higher nutrition will render possible
a greater propagation, sexual or asexual, and this may be abundantly veri-
fied by observation and experiment. Witness the case of aphides, in which
the rate of jxirthenogenelic reproduction is found to be directly proportional
to temperature and food-supply ; or, again, that of domestic animals, such as
the sheep, whose fertility is in direct relation to richness of pasture and
warmth of climate ; or, finally, and most obviously of all, that of field or
fruit crops, upon which the influence of increased liberality of manuring
will not be disputed. Yet it is sometimes maintained, for both plants and
animals, that overfeeding checks increase, while limited nutriment stimu-
lates it ; and to support this view there are cited such cases as that of the
Ixirrenness of a very luxuriant plant, and the fruitfulness which appears on
its depletion. But if this objection really held, manuring would in all cases
be inexpedient, instead of only in plants where the growth of sexless axes
LAWS OF MULTIPLICATION.
303
is still too luxuriant ; and a tree which has borne a heavy crop should, by
this depletion, bear again yet more heavily, instead of being more or less
barren next year unless manured. Or the difficulty may also Ije met by
interpreting such vegetative luxuriance, not as a case of
higher individuation at all, but simply as a case of asexual
multiplication of secondary axes ; or again, and perhaps
most simply, by regarding the appearance of sexual re-
production on depletion simply as a case of the previously
demonstrated antagonism between genesis and growth.
But again, since fatness is associated with sterility, it g,
is.often argued that high feeding is unfavourable to gene-
sis. Obesity, however, is now known to be associated h—
with imperfect assimilation, with physiological impoverish-
ment or degeneration, — by no means with that constitu-
tional wealth which is favourable to fertility. If, in short,
we bear in mind that truly high nutrition means only due
abundance of, and due proportion among, all the sub-
stances which the organism requires, and that their per-
fect assimilation by the organism is also needful, such
objections to the generalisation not only disappear, but
such a phenomenon as the coincidence of returning fer-
tility with disappearing obesity affords a confirmatory
argument.
Organisms having aberrant modes of life are next ap-
pealed to for crucial evidence bearing on these general
doctrines. Thus, turning to vegetable and animal para-
sites, which combine superabundant nutrition with greatly
diminished expenditure, the enormous fertility exhibited
by all such forms is seen to be the necessary correlative
of such a state of nutrition and expenditure, and not
merely an acquired adaptation to their peculiar difficulties
of survival. The reversion exhibited by so many species
(especially among the higher arthropods, e.^^.. Aphis,
Cecidof/iyia) from sexual reproduction to primitive forms
of genesis, is explained by pointing out that such species
are peculiarly situated in obtaining abundant food with
little exertion. Among bees, ants, and termites alike,
the enormous fertility of the inactive and highly nourished
qucen-mQther are obviously also cases in point.
The inverse variation of genesis with individuation has
now been demonstrated inductively as well as deductively,
and that for each element of the latter (growth, develop-
ment, or activity). Yet before discussing its application
to the problems of the multiplication of the human species,
two points remain, — a question has to be answered, and
a qualification made. The question, only partially i^ species of Onion
answered in course of the preceding argument, is, How is with asexual vege-
the ratio between individuation and genesis established in tative l>">bils (/^)
each special case ? and the answer is, By natural selec- ?J?^"^ ' * owers
tion. This may determine, whether the quantity of
matter spared from individuation for genesis be divided into many small ova
or a few larger ones ; whether there shall be small broods at short intervals,
304 THE EVOLUTION OF SEX.
or lai^er broods at longer intervals ; or whether there shall lie many unpro-
tected offspring, or a few carefully protected by the parent. Again, survival
of the fittest has a share in determining the proportion of matter subtracted
from individuation for genesis. Yet this operation of natural selection goes
on strictly under the limits of the antagonism above traced.
The needed qualification arises on mtroducing the conception of evolu-
tionary chance. If time be left out of account as hitherto, — or, what is the
same thing, if all the species be viewed as permanent, — the inverse ratio
between individuation and genesis holds absolutely. But each advance in
individual evolution (it matters not whether in bulk, in structure, or in
activities) implies an economy ; the advantage must exceed the cost, else it
would not be perpetuated. The animal thus becomes physiologically
richer; it has an augmentation of total wealth to share between its in-
dividuation and its genesis. And thus, though the increment of individua-
tion tends to produce a corresponding decrement of genesis, this latter will
be somewhat less than accurately proportionate. The product of the two
factors is greater than before ; the forces preservative of race become
greater than the forces destructive of race, and the species spreads. In
short, genesis decreases as individuation increases, yet not quite so fast.
Hence every type that is best adapted to its conditions — every higher
type — has a rate of multiplication that ensures a tendency to predominate.
For though the more evolved organism is the less fertile absolutely, it is
the more fertile relatively.
The whole generalisation admits of the simplest graphic
illustration. For if the line AB represents the aggregate
C
A ' B
matter or energies, the structures or the functions, of the
organism, of which AC denotes the amount devoted to in-
dividuation and CB to reproduction, the inverse variation of
AC to CB is obvious, as also if AC and CB represent the
psychological obverse of these two classes of function. Nor
does an increase in total energy modify this, as when the
stronger members of a species frequently also exhibit greater
reproductive power ; for if in one case AB = 20, of which
CB = 4, and in another AB = 25, CB may become 5 without
any rise of reproductive ratio, since -^j^ = ^j. But if the species
be evolving, the advance in individuation implies a certain
economy, of which a share may go to diminish the decrement
to genesis, as above explained.
§ 4. Spencer's Application of his Results to Man, — In ex-
tending this hard-won generalisation to the case of man, the
concomitance of all but highest total individuation with all but
lowest rate of multiplication (the enormous bulk of the elephant
involving a yet greater deduction from genesis) is at once
apparent. Comparing different races or nations, or even
LAWS OF MULTIPLICATION.
305
different social castes or occupations, the same holds good;
while the prevalence of high multiplication in races of which
the nutrition is in obvious excess over the expenditure is also
evident, witness the Boers or French Canadians. Such an
apparent difficulty as that of the Irish, in whom rapid multipli-
cation occurs despite poor food, is accounted for by the re-
latively low expenditure in obtaining it (since the " law of
diminishing return " implies its converse for diminishing labour),
though, no doubt, also in part by the habit of early marriage, if
not by some measure of lowered- individuation as well. The
main position being established, Spencer proceeds to discuss
the question of human population in the future, and insists
strongly on the importance of pressure of population, which he
regards as the main incentive to progress alike in past, present,
and future. Reviewing the possibilities of progress in bulk,
complexity of structure, multiplication and variation of func-
tion, he concludes that the more complete moving equilibrium,
and more perfect correspondence between organism and
environment, which such evolution involves, must take place
mainly in the direction of psychical development. Yet this
development, while stimulated by pressure of population, con-
stantly tends to diminish the rate of fertility ; in other words,
this cause of progress tends to disappear as it achieves its full
effect. The acute pressure of population, with its attendant
evils, thus tends to cease as a more and more highly individu-
ated race busies itself with its increasingly complex yet normal
and pleasurable activities, its rate of reproduction meanwhile
descending towards that minimum required to make good its
inevitable losses.
§ 5. Summary of the Population Question. — The general
question, so far as yet developed, may now be conveniently
summarised in the accompanying tabular form. Here the
stage of knowledge reached by each author, together with
any practical applications therefrom deduced, may be read
horizontally, while the historic development of each separate
line of conceptions may be traced vertically.
From such a summary, brief as it is, the main steps in the
development of our knowledge are clear enough, but a deeper
analysis is required before final exposition or complete appli-
cation is possible. Nor, when we note how vast the progress
of science through the advance in precision and extension
20
3o6
THE EVOLUTION OP SEX.
effected upon the conception of Malthus* by Darwin, will the
utility of such increasing elaboration be disputed. Thus the
full inductive verification of Spencer*s law involves a detailed
Author.
Development of Theory of Population.
Practical
Action
Deduced.
1.
Non -bio-
logical
writers
(prede-
cessors
and op-
ponents
of Mal-
thus).
II.
Malthus.
1793.
III.
Darwin.
1859.
Increase of population
does not tend to out-
run subsistence.
1
Increase of population ■ But meets checks:
tends to outrun that ' A. Positive,
of subsistence. B. Preventive.
To avoid A,
adopt H.
Do.
Hence _ struggle
for existence :
A. - Natural
selection.
B. Artificial
selection.
Leading to
evolution.
Laissfz -fairt^
i.e.y on ac-
count of ad-
vantage to
species from
A, avoid B.
IV.
Spencer.
1852-66.
Do.
Rate of multiplication
invest igated^ for dif-
ferent species, and
shown to vary inverse-
ly as individuation.
Do.
Do.
Also lead-
ing to
e v 1 u-
tion of
species.
Do.
\lndividiiaie.'\
comparison of the rates of reproduction of each group of
organic species, with their observed degree of individuation
(first in each of its factors, and finally in their sum), devia-
tions from the inverted symmetry of the theoretic curves
(see fig. opposite) having to be separately discussed. Natural
selection also requires a yet deeper analysis ; the limits and
possibilities of artificial selection are but little known, while
* It is also interesting to compare Malthus's view of txtpulation, tend-
iiipj to increase in geometrical proportion and substance only in arithmetical,
with Spencer's demonstration of the limit of growth already summarised
(see p. 220), the more so when we bear in mind that reproduction is dis-
continuous growth. The precise statement of Malthus becomes confirmed,
as regards the cell, if not the cell aggregate.
307
a theory of variation is still far from agreed upon. If how-
ever we bear in mind that the amount of evolution in given
time is but small our knowledge seems not insufficient for the
practical deductions which are so pressingly demanded ; yet it
is here that the most serious disagreement has prevailed.
'I'hus the Malthusian position is obviously inadequate, in not
allowing for the Darwinian one ; yet the converse also is
undeniable, for the posiiion of laissez-faire, upon which Darwin
and Spencer alike take their stand, not
only almost ignores the wellbeing of the
individual in considering the advance-
ment of the species, but is even then
too optimistic, since it not only fails
to accelerate the progressive evolution
which is alone considered, but also fails
to provide against the equal possibility
of degenerative change. Are we then
simply to return to the somewhat crude
proposals and excessive hopes for the
increase of individual wellbeing due to
Malthus or his followers, based too as
these have been on imperfect pre-
Spencerian knowledge ?
The answer is not far to seek, — it
- lies in the generalisation above estab- Leiihenrpcndicuiuiabdwitu
iished;-yet it is remarkable that Mr di!J™onouV£SividS^LDS
Spencer, alter not only establishing the of « wrics of fonni i, > ,, ,,
inverse variation of individuation and Sird. IJan.'^itphSi^i), ^J
genesis among species in general, but f^Ji'^'^ i"rt'^° "TS^Jwo
even showing for the human species in Df'muitipMcIi^^ihe'uiBe
particular that it is essentially upon (i^ ■iJo^wrfcTof^tiSnis
increase of the psychical activities that r«i«cii.tiy[iiintr»i(biih.ii
the increased mdividuation and dimin- rai'La"Vi"divSSi[i™""™
ished genesis of the future must depend, v^-:»-
should not have proceeded to a fuller application. For unless
the main generalisation be abandoned, it is obvious that the
progress of the species and of the individual alike is secured and
accelerated whenever action is transferred from the negative
side of merely seeking directly to repress genesis, to the
positive yet indirect side of proportionally increasing individua-
tion. This holds true of all s|)ecies, yet most fully of man,
since chat modilication of i>sychical activities in which his
3o8 THE EVOLUTION OF SEX.
evolution essentially lies, is par excellence and increasingly the
respect in which artificial comes in to replace natural selection.
Without therefore ignoring the latter, or hoping ever wholly
to escape from the iron grasp of nature, we yet have within our
power more and more to mitigate the pressure of population,
and that without any sacrifice of progress, but actually by
hastening it. Since then the remedy of pressure and the hope
of progress alike lie in advancing individuation, the course for
practical action is clear, — it is in the organisation of these
alternate reactions between bettered environment (material,
mental, social, moral) and better organism in which the whole
evolution of life is defined, in the conscious and rational
adjustment of the struggle into the culture of existence.
The practical corollaries of the Malthusian view are celibacy,
late marriage, and moral control ; the objections are vice, in-
creased mortality in childbirth, and the present low evolution
of our moral nature. The practical corollary of the Darwinian
doctrine is virtually nil ; the objection, that the survival of
what we consider the best types is doubtful, and that the
survival of the fit is apt to be cruel. The practical corollaries of
the Spencerian principle, although Mr Spencer can hardly be
said to have insisted upon these, are individuate and educate.
The objection is, that the pressure of population is already felt,
and that individuation is a matter of centuries. Furthermore,
the effect of education, for instance in reducing sexuality, will
tell most where it is least wanted, viz., among the best types.
We are therefore bound to include, as a continuation of the
above table, the amendment of some of the most thoughtful ex-
ponents of what is generally called neo-Malthusian doctrine.
This advocates the use of artificial preventive checks to fer-
tilisation. Discussion of this proposal is at present difficult,
because of the comparative absence of distinctly expressed
opinion on the part of medical experts, and because of strong
superficial prejudices, not only against the scheme, but against
its discussion. These prejudices are, however, dying out, and
that is well, for they do nothing but obscure appreciation alike
of the merits and demerits of the doctrine. An increasing
realisation of the plain facts of reproduction and population
must rapidly exterminate the persistently theological absurdities
which people utter, if they do not believe on the subject. The
vague feeling that control of fertilisation is " interfering with
nature," in some utterly unwarrantable fashion, cannot be
LAWS OF MULTIPLICATION. 309
consistently stated by those who live in the midst of our highly
artificial civilisation. The strongest prejudice seems to be
based in a moral cowardice, which gauges a scheme by its
" respectability," while even more culpable is that consciously
or unconsciously derived from the profitableness to the
capitalist classes of unlimited competition of cheap unskilled
labour. For never did the proletariat more literally deserve its
name than since the advent of the factory period, their rapid
and degenerative increase, indeed, primarily representing " the
progress of investments."
The general attitude of the modern Malthusian may first of
all be roughly indicated by quoting the mottoes which head
the organ of their league. " To a rational being, the prudential
check to population ought to be considered as ec^ually natural
with the check from poverty and premature mortality" (Malthus,
1806). " Little improvement can be expected in morality until
the production of large families is regarded in the same light as
drunkenness, or any other physical excess " (John Stuart Mill,
1872). " Surely it is better to have thirty-five millions of
human beings leading useful and intelligent lives, rather than
forty millions struggling painfully for a bare subsistence "
(Lord Derby, 1879). Starting from the familiar induction
that "population has a constant tendency to outrun the
means of subsistence," they recognise in this over-population
" the most fruitful source of pauperism, ignorance, crime,
and disease." To counteract this there are checks, posi-
tive or life-destroying on the one hand, prudential or birth-
preventing on the other, " The positive or life-destroying
checks comprehend the premature death of children and adults
by disease, starvation, war, and infanticide." As these positive
checks are happily reduced with the progress of society,
attention must be concentrated on the other side. " This
consists in the limitation of offspring by abstention from
marriage, or by prudence after marriage." But as to the first,
prolonged abstention from marriage, as advocated by Malthus,
this is " productive of many diseases, and of much sexual vice,"
while ** early marriage, on the contrary, tends to secure sexual
purity, domestic comfort, social happiness, and individual
health." The check that remains to be advocated is thus
"prudence after marriage," and by this the neo-Malthusians
most distinctly mean attention to methods which will secure
that sexual intercourse be not followed by fertilisation. For
3IO THE EVOLUTION OF SEX.
the details of the various methods, we must refer to the
Malthusian literature ; but a brief outline is imperative, even
for an approximate understanding of the problem.
(a.) Thus we have the suggestion that intercourse should be
limited to the relatively infertile period most remote from
menstruation, when conception may indeed occur, but with
less probability than at other periods. Although gynaecologists
are disagreed as to the degree of this probability, there can be
little doubt that such limitation would have a useful influence,
although in itself confessedly incomplete. The so-called
artificiality of control is here reduced to a minimum, and the
suggestion is obviously in harmony with that increased
temperance which all must allow to be desirable.
(d.) In the second place, there are methods employed by
the males, such as that of withdrawal before the emission
of the seminal fluid, a habit common enough both in savage
and civilised communities. Fertilisation is in this way ab-
solutely prevented, but apart from a more general objection to
be afterwards emphasised, such a practice is maintained by
some to be injurious to the male, and yet more to the female.
Moreover, although the risks of over-p>opulation and female
exhaustion by child-bearing are here minimised, there is still
risk of male exhaustion.
(c) Thirdly, although again under the severe criticism of
some of the medical experts, there are means employed by the
females, for securing by means of pessaries that the spermatozoa
do not come into contact with the ovum, or by means of washes
that the male elements are rendered ineffectual. In reply to
the medical objections to both these methods of artificial check,
it is answered (a) that it may in many cases be necessary to
choose between two evils, of which the risk involved in the
artificial check may he much less than that involved in con-
tinued child-bearing ; (/y) that it is hardly a fair argument as
yd to urge that the proposed checks of neo-Mahhusianism
arc fraught with danger. As to the popularly supposed pre-
ventive check of prolonged nursing one baby in the hope of
thereby preventing a new conception, it is necessary to em-
phasise that nursing does not effect this, and that the prolonga-
tion of the lacteal function and diet beyond their natural limits
is seriously injurious alike to mother and offspring.
Even recognising spme of these objections, the neo-Malthu-
sians urge the number of distinct advantages, — the reduction
LAWS Of muLtiplicatioi*?. 311
of the present rapid rate of increase ; the possibility of earlier
marriages, and a probable diminution of vice ; an increase in
the fitness of the race by lessening the propagation of unfit
types and the exhaustion of the mothers by too frequent child-
bearing. Supposing, again, the general adoption of the pro-
posal, the neo-Malthusians insist upon the possibility of a
heightened standard of comfort among the poorer members
of the community, and the removal of obstacles to marriage
which stand in the way of those who ought to marry but ought
not to be parents.
Without urging medical objections above referred to, —
for in regard to the discussion of these, professional experts
must bear the responsibility, — we must emphasise several
counter-arguments. Thus it has been maintained, though with
no great degree of certitude, that a proposal involving some
deliberate and controlled action would tend to be adopted
most where least wanted, viz., among the more individuated
types, whose numbers would in consequence be proportionately
reduced. The diminished rate of increase, which is the most
obvious social result of the extensive adoption of neo-
Malthusian practices, has long been known to the student of
population; and in some countries, particularly France, —
although here, no doubt, to some extent the result of peculiarly
high individuation, — is a recognised national danger, especially
since the diminished population, in being largely freed from the
normal acuteness of the struggle for existence, loses many of
the advantages of this as well.
The statistician will doubtless long continue his fashion of
confidently estimating the importance and predicting the sur-
vival of populations from their quantity and rate of reproduction
alone ; but at all this, as naturalists we can only scoff. Even
the most conventional exponent of the struggle for existence
among us knows, with the barbarian conquerors of old, that "the
thicker the grass, the easier it is mown ; " that " the wolf cares
not how many the sheep may be." It is the most individuated
type that prevails in spite, nay, in another sense, positively
because of its slower increase ; in a word, the survival of a
species or family depends not primarily upon quantity, but
upon quality. The future is not to the most numerous popu-
lations, but to the most individuated. And as we increas-
ingly see that natural history must be treated primarily from
the standpoint of the species-regarding sacrifice rather than
312 THE EVOLUTION OF SEX.
from that of the individual struggle, we see the importance of
the general neo-Malthusian position, despite the risks which the
particular modes of its practice may involve.
Apart from the pressure of population, it is time to be learn-
ing (i) that the annual childbearing still so common, is cruelly
exhaustive to the maternal life, and this often in actual duration
as well as quality; (2) that it is similarly injurious to the
standard of offspring ; and hence (3) that an interval of two
clear years between births (some gynaecologists even go as far
as three) is due alike to mother and offspring. It is time there-
fore, as we heard a brave parson tell his flock lately,. " to have
done wnth that blasphemous whining which constantly tries to
look at a motherless " (ay, or sometimes even fatherless) " crowd
of puny infants as a dispensation of mysterious providence."
Let us frankly face the biological facts, and admit that such
cases usually illustrate only the extreme organic nemesis of
intemperance and improvidence, and these of a kind far more
reprehensible than those actions to which common custom
applies the names, since they are species-regarding vices, and
not merely self-regarding ones, as the others at least primarily
are. To realise the social consequences of sexual intemperance
is enough to obviate any hasty criticism of neo-Malthusianism,
whatever conclusion may be arrived at as to its sufficiency.
It is time, however, to point out the chief weakness in neo-
Malthusian proposals, which are at one in allowing the gratifica-
tion of sexual appetites to continue, aiming only at the preven-
tion of the naturally ensuing parentage. To many doubtless
the adoption of a method which admits of the egoistic sexual
pleasures, without the responsibilities of childbirth, would mul-
tiply temptations. Sexuality would tend to increase if its respon-
sibilities were annulled ; the proportion of unchastity before
marriage, in both sexes, could hardly but be augmented ;
while married life would be in exaggerated danger of sinking
into " nionogamic prostitution." On the other hand, it seems
probable that the very transition from unconscious animalism
to deliberate prevention of fertilisation, would tend in some to
decrease rather than increase sexual appetite.
It seems to us, however, essential to recognise that the ideal
to be sought after is not merely a controlled rate of increase,
but regulated married lives. Neo-Malthusianism might secure
the former by its more or less mechanical methods, and there
is no doubt that a limitation of the family would often increase
LAWS OF MULTIPLICATION. 313
the happiness of the home ; but there is danger lest, in re-
moving its result, sexual intemperance become increaisingly
organic. We would urge, in fact, the necessity of an ethical
rather than of a mechanical " prudence after marriage," of a
temperance recognised to be as binding on husband and wife
as chastity on the unmarried. When we consider the inevit-
able consequences of intemperance, even if the dangers of too
large families be avoided, and the possibility of exaggerated
sexuality becoming cumulative by inheritance, we cannot help
recognising that the intemperate pair are falling towards the
ethical level of the harlots and profligates of our streets.
Just as we would protest against the dictum of false physi-
cians who preach indulgence rather than restraint, so we must
protest against regarding artificial means of preventing fertilisa-
tion as adequate solutions of sexual responsibility. After all, the
solution is primarily one of temperance. It is no new nor
unattainable ideal to retain, throughout married life, a large
measure of that self-control which must always form the organic
basis of the enthusiasm and idealism of lovers. But as old
attempts at the regulation of sexual life have constantly fallen
from a glowing idealism into pallor or morbidness, it need
hardly be said that the same fate will ever more or less
befall the endeavour after temperance, so long as that lacks
the collaboration of other necessary reforms. We need a
new ethic of the sexes ; and this not merely, or even mainly,
as an intellectual construction, but as a discipline of life ;
and we need more. We need an increasing education and
civism of women, — in fact, an economic of the sexes very
different from that nowadays so common, which, while attack-
ing the old co-operation of men and women because of its
manifest imperfections, only offers us an unlimited and far
more mutually destructive industrial competition between them
instead. The practical problems of reproduction become in
fact, to a large extent, those of improved function and evolved
environment; and limitation of population, as we are begin-
ning to see in regard to the more individual forms of intem-
perance, is primarily to be reached, not solely by individual
restraint, but by a not merely isolated and individual, but aggre-
gate and social, reorganisation of life, work, and surroundings.
And while our biological studies of course for the most part
only point the way towards deeper social ones, they afford also
one luminous principle towards their prosecution, — that thorough
314 THE EVOLUTION OF SEX.
parallelism and coincidence of psychical and material considera-
tions, upon which moralist and economist have been too much
wont respectively to specialise.
§ 6. Rate of Reproduction^'''' Nir^ — Sterility, — When we view
reproduction in terms of discontinuous growth, — that is, as a
phenomenon of disintegration, — it is obvious that complete
integration of the matter acquired by the organism into its own
bulk, and for its own development, precludes reproduction, —
that is, involves sterility, — and similarly as regards the energies
of the organism. This is only a re-statement of Spencers
generalisation above discussed ; for it is evident that, if genesis
vary inversely as individuation, it must be suppressed altogether
if individuation becomes complete. The actual phenomena,
however, by no means usually admit of explanation as such
realisations of the ideal of evolution, and hence the cause and
treatment of sterility mainly pass into the provinces of the
experimental naturalist and the physiological physician. From
the earliest times, indeed, physician and naturalist, priest and
legislator, alike devoted attention to the subject ; and it was
probably in this way, as a recent monographer remarks, that
research became directed to the larger problem of repro-
duction in general. The general biological questions —
e.g,^ the relations between sterility within the limits of
a species to changes in the environment, or that of sterility
among hybrids — are extensively discussed in the copious
literature which centres around Darwin's "Variation of Animals
and Plants under Domestication " ; while with regard to the
human species, an extensive medical literature of course exists,
to which any encyclopaedia of medicine, or conveniently the
recent careful monograph of P. Miiller (" Die Unfruchtbarkeit
der Ehe," Stuttgart, 1885), will furnish bibliographical details.
LAWS OF MULTIPLICATION. 315
SUMMARY.
J, The rate of reproduction is chiefly determined by the constitution of
the crganism ; the rate of increase, by its relations to the animate and
inanimate environment.
2. The naturalist has to thank the sociologist for directing emphatic
attention to the laws of multiplication.
3. Summary of Spencer's analysis. Individuation and genesis vary
inversely.
4. In regard to man, Spencer urges the importance of pressure of popu-
lation as an incentive to progress, and concludes that man's future evolution
must continue mainly in the direction of psychical development, and pre-
dicts with the increase of individuation a diminution of fertility.
5. Predecessors and opponents of Malthus denied that increase of
population tended to outrun subsistence ; Malthus successfully demon-
strated his thesis, and noted the checks which curbed the increase ; Darwin
emphasised the advantage of the pressure and checks ; Spencer shows the
inverse ratio, of degree of development and rate of reproduction ; neo-
Malthusians advocate the use of artificial preventive checks to fertilisation.
Discussion of these various generalisations and proposals.
6. Completed individuation, were that possible, would be theoretically
associated with sterility.
LITERATURE.
Malthus. — Theory of Population. 1806.
Spencer. — Principles of Biology. Lond. 1866.
Geddes. — " Reproduction," Ency. Brit. ; and Lecture on Claims of
Labour. Edin. 1886.
Drysdale. — The Population Question. Lond. 1878.
Besant. — The Law of Population. Lond. n.d.
Clapperton. — Scientific Meliorism. Lond. 1885.
CHAPTER XXI.
The Reproductive Factor in Evolution.
§ I. General history of Evolution, — The history of the doctrine
of evolution is essentially modern ; for though the idea glim-
mered before the minds of many ancient philosophers from
Empedocles to Lucretius, it was not till the eighteenth century
that naturalists began seriously to apply the conception to the
problem of the origin of our fauna and flora. In thinking of
the history, it is necessary to distinguish, on the one hand, the
gradual growth of the conviction that the theory of evolution
is a satisfactory modal interpretation of the origin of animate
nature as we know it, and, on the other, the inquiry into the
real mechanism of the process. The value of the evolution
doctrine as a thought-economising formula was made quite
clear by the labours of Spencer, Darwin, Wallace, Haeckel,
and others j the real aetiology of organisms, the " how " of the
evolution process — is still the subject of searching inquiry and
keen debate.
The idea of evolution, for so many centuries a latent germ,
first took definite shape, so far as biology is concerned, in the
mind of BufTon (1749), who not only urged the general con-
ception with diplomatic skill and powerful irony, but sought to
elucidate the working out of the process. He illustrated the
influence of new conditions in evoking new functions; showed
how these in turn reacted upon the structure of the organism ;
and how, most directly of all, changes of climate, food, and
other elements of the environment, were external factors
evoking internal change, whether for progress or for degene-
ration.
Contrasted with Buflbn in many ways, both in his mode of
treatment and in his view of the factors, was Erasmus Darwin
( 1 794), the grandfather of the author of the " Origin of Species."
In rhyme and reason, with all the humour and common-sense
of a true Englishman, and with a vivid appreciation of life as
THE REPRODUCTIVE FACTOR IN EVOLUTION. 317
more than mechanisn], he stated the general conception of
evolution, and emphasised the organism's inherent power of
self-improvement, the moulding influence of new needs,
desires, and exertions, and the indirect action of the environ-
ment in evoking these.
*To Treviranus (writing in 1802-31) — a biologist too much
neglected both in his lifetime and since — organisms appeared
almost indefinitely plastic, especially however under the direct
influence of external forces. His keen analysis of possible
factors did not fail to recognise — what Brooks, Gallon, Weis
mann, and others have since elaborated — that the union of
diverse sexual elements in fertilisation was in itself a fountain
of change. " Every form of life," he says, " may have been
produced by physical forces in either of two ways, either from
formless matter, or by the continuous modiflcation of form.
In the latter case, the cause of change may be either in the
influence of the heterogeneous male reproductive matter on the
female germ^ or in the influence of other potencies after
generation."
His contemporary Lamarck (writing in 1801-9)— of greater
posthumous fame — fought in poverty like a hero for the evolu-
tionary conceptions of his later years. He is well known to
have emphasised the importance of changed conditions in
evoking new needs, desires, and activities, urging at the same
time the perfection wrought upon organs by increased practice,
and conversely the degeneration which follows as the nemesis
of disuse. As regards the evolution of plants, he laid the main
emphasis on the modifications brought about by the environ-
ment. Evolution seemed to him to be due to the interaction
of two fates, — an internal progressive power of life ; and the
external force of circumstances, encountered in the twofold
struggle with the inanimate environment and with living
competitors. The keynote of his system was that adaptive
modifications in the bodies of organisms are brought about by
changes in function or in environment, or in both, and that
these modifications are in some degree at least inheritable.
Among the philosophers too, and especially in the minds
of those who had been disciplined in physical or historical
investigations, the speculations of the ancients were ever taking
fresh form, gaining moreover in concreteness. Thus Kant
viewed the evolution of species mainly in terms of the
mechanical laws of the organism itself, but allowed also for
31 8 THE EVOLUTION OF SEX.
the influence of environment, noted the importance of selection
in artificial breeding, and, like such ancients as Empedocles
and Aristotle, had glimpses of the notion of the struggle for
existence. The same idea is more distinct in Herder's
** Philosophy of History," where, probably under Goethe's
influence, he speaks of the " struggle, each one for itself, as if
it were the only one," of the limits of space, and of the gain to
the whole from the competition of individuals. Oken (1809)
saw the light of the evolution idea dancing like a will-o'-the-
wisp in the mist of his '^Urschleim" speculations, and seemed
chiefly to interpret the organic progress in terms of action and
reaction between the organism and its surroundings ; while in
the noble epic of evolution which we owe to his contemporary
Goethe, the adaptive influence of the environment and the
inherent growth-tendencies of the organism are especially
emphasised.
Wells in 181 3, and Patrick Matthew in 183 1, forestalled
Darwin in suggesting the importance of natural selection ; but
their virtually buried doctrines, however interesting historically,
were of less practical importance than those of Robert
Chambers, the long unknown author of the "Vestiges of
Creation " (1844-53). His hypothesis of evolution emphasised
the growing or evolving powers of the organisms themselves,
which developed in rhythmic impulses through ascending
grades of organisation, modified at the same time by external
circumstances, which acted with most eflect on the generative
system. It is difficult indeed to refrain from amusement or
irritation at the naive simplicity with which he evolves a
mammal from a bird, by the short and easy method of pro-
longing the period of uterine life in favourable nutritive condi-
tions ; but though a goose could not so simply give rise to a
rat, the emphasis laid on the influence of prolonged gestation
is full of suggestion. Apart from his common-sense view of
evolution as a process of continued growing, Chambers
deserves to be remembered as one of the first to appreciate
"the force of certain external conditions operating upon the
parturient system."
In France, Etienne and Isidore Geofl*roy St Hilaire — father
and son — denied indefinite variations, regarded function as of
secondary importance, and laid special stress upon the direct
influence of the environment. To them it seemed not so
much the effort to fly, as the (supposed) diminished proportion
THE REPRODUCTIVE FACTOR IN EVOLUTION. 319
of carbonic acid in the atmosphere, which had determined the
evolution of birds from ancient reptiles. A complete history
of evolution theories, up to the publication of the " Origin of
Species" (1859), would have to take account further of the
opinions of the geographer Von Buch and the embryologist
Von Baer, of Schleiden and Naudin, Owen and Carus, and
many others ; but no such survey is here our purpose.
For it must be already evident from the above brief sketch
of representative opinions, that successive naturalists have
emphasised now one factor and now another in the evolu-
tionary process. To one it seemed as if the organism had a
motor power of development — often a metaphysical one, it
must be allowed — within itself, and that evolution was to be
explained, in Topsian fashion, *' according to the laws of
organic growth;" to another, function appeared all-important,
perfecting organs on the one hand, allowing them to wane in
disuse on the other; to a third, organisms were seen under
the hammers of external forces and circumstances, being con-
tinuously welded into more and more perfectly adapted forms.
The intrinsic character of the organism, its function, and its
environment, on each of the three factors emphasis was in turn
laid.
At this juncture Darwin elaborated his theory of "The
Origin of Species by means of Natural Selection and the
Preservation of Favoured Races in the Struggle for Life," and
was independently and simultaneously corroborated by Alfred
Russel Wallace. They did not indeed deny a spontaneous
power of change in the organism itself, nor the influence of
function and environment; but, without definitely discussing
the origin of variations, sought to show how the destructive or
eliminating, and the conservative or selecting agency of the
animate and inanimate environment, were the directive factors
in evolution. Given a sufRcient crop of indefinite variations,
— unanalysed or unanalysable as to their origin, — the struggle
for existence separated the minority of wheat ears from the
majority of tares, and secured a finer and finer harvest.
So much had Darwin in his magistral labours to do with
making the general conception of evolution current coin, that
we can readily understand how not only the educated laity,
but the majority of professed naturalists, identified their
adherence to the general doctrine with a subscription to the
specific principle of natural selection, and in becoming evolu-
320 THE EVOLUTION OF SEX.
tionists became at the same time Darwinians, that is to say,
natural selectionists. Of late years, however, as conflict has
passed from the outworks to the very citadel of evolution, — has
come, that is to say, to centre round the problem of the origin
of variations, — history has repeated itself. Naturalists such as
Nageli, Mivart, and Eimer have championed the cause of
internal organismal variations, of evolution in terms of the con-
stitution of the oiganism, of progress according to the definite
laws of organic growth. An active school of neo-Lamarckians,
such as Cope and Packard, has arisen in America; while
Spencer has re-emphasised the importance both of function and
of environment as factors in organic evolution, supported more-
over in this position by the experimental work of Semper and
others. The last published essays of Spencer may be referred
to in illustration of the unended state of the controversy, but
at the same time, of the growing tendency to limit the importance
of natural selection, and as a good instance of successful
endeavour to recognise the measure of truth in the different
theories. Wallace remains staunchest among the upholders of
the theory of natural selection, for his share in which he seems
ever to refuse to take to himself sufficient credit; but it is
interesting to notice, that in his '' Darwinism" (1889), in re-in-
forcing his old objections against the importance which Darwin
attached to sexual selection, he has made admissions welcome
to those of us who believe that the shoulders of natural selection
have also been overburdened. As we have already noticed,
the phenomena of male ornament are discussed and summed
up as being **due to the general laws of growth and develop-
ment," so that it is *' unnecessary to call to our aid so
hypothetical a cause as the cumulative action of female pre-
ference." Again, *' if ornament is the natural product and direct
outcome of superabundant health and vigour," — a view to which
the reader of the preceding pages can be no stranger, — ''then
no other mode of selection is needed to account for the presence
of such ornament" But if the origin of characters so im-
portant as those often possessed by males is to be ascribed
to internal constitution rather than to the external selection of
indefinite variations, the suggestion seems obvious that the
origin of this, that, and the other set of characters may also be
explained in the same way. A vivid historical account of the
evolution of evolution-theory will be found in Osborn's " From
the Greeks to Darwin," but a much larger work will be neces-
THE REPRODUCTIVE FACTOR IN EVOLUTION. 31!
saiy if justice is to be done to many who have contributed to
working out the most characteristic idea of the nineteenth
cemury. Thus St uirt-Glen die's insight in seeking to bring
the laws of inorganic processes into line with the processes
of organic evolution has never received due recognition.
Before we conclude this historical sketch, we must however
refer to the subject of debate re-opened by Weisraann, to
whom, as one of the foremost of European naturalists, the
reader's attention has already been so frequently directed To
a very large extent at least, we and our fathers have believed
that characters acquired by the individual organism from
Iwo adjaccnl Animjtl cells, showing cominuniationii throttsh
adJAcenI intercellular sutelance ; also the protopLunic
ncEwork, jtnd th« nudcm- — AlW P6tzii*T.
functional or environmental conditioiis might be transmitted as
a legacy to the offspring. According to Weismann, and not a
few others independent of and dependent on him, this has
been a delusion. Not only is positive proof of such transmis-
sion of acquired characters, i.e., other than those of constitu-
tional, congenital, or germinal origin, so scanty and unsatisfactory
that His has not hesitated to call the catalogue of cases a mere
" handful of anecdotes," but the connection between the body-
cells and the sex-elements seems to Weismann and his school
so far from close or dependent, that there is a great probability
322 THE EVOLUTION OF SEX.
against any " somatic " modification specifically and represen-
tatively affecting the reproductive dements, — or, what comes to
the same thing, the offspring. If the reproductive elements, in
spite of the close connection between all parts of the body,
or even between cell and cell (see above fig.)* are unaffected
directly and specifically by changes in the other parts of the
body, then the functional and environmental " mo4ifications "
of the body, however important to the individual, are only of
indirect importance in the evolution of the race. It has been
suggested by Baldwin, Osborn, and Lloyd Morgan that advan-
tageous modifications may serve in the struggle for existence as
an individual shield until such time as congenital and therefore
transmissible variations in the same direction may be estab-
lished; but even if this be demonstrated, it remains true that
the evolutionary importance of modifications is indirect. If
individually acquired characters or " modifications " are of
importance only to the individual body, they are obviously of
no direct moment in the evolution of the species, — above the
level of the Protozoa at least; and, as Weismann himself says,
the ground is thus taken from under the feet of Buffonians,
Lamarckians, neo-Lamarckians, etc The ground is left clear for
natural selectionists, and the struggle for existence acting on
variations remains as the chief factor in the mechanism of
evolution. Though we cannot demonstrate that a logical possi-
bility has been the t^ra causa of past evolution, much has been
done to establish the probability. But much still requires to
be done by actual observation in the present to show that dis-
criminate elimination is of frequent occurrence, for it is on the
assumption of discriminate, as opposed to indiscriminate, elimi.
nation that the case for natural selection rests. To the ques-
tion. What starts these variations which natural selection
eliminates or fosters? Weismann has suggested various
answers : — {a) That the action of the environment on the bodi-
less Protists established a multitude of differences of which all
subsequent variations in the multicellular organisms are simply
permutations and combinations; (p) that a prolific source of
variation is to be found in the intermingling or amphimixis of
the sex-cells in fertilisation, and in the reduction-processes
associated with the maturation of these sex-cells; and {c) that
the . germ-plasm is provoked to vary by nutritive and other
stimuli acting on it from without, or from the enclosing body
of the parent-organism. There are many, however, who would
THE REPRODUCTIVE FACTOR IN EVOLUTION. 323
Still say, that even if none but constitutional or germinal varia-
tions are transmissible, we are not shut up to the exclusive
adoption of the natural selectionist position. It is still open
to the naturalist to demonstrate that many adaptations at least
are not explicable as the result of a long process of fostering
and eliminating selection among a host of sporadic indefinite
variations, but are rather the direct and necessary results of
** laws of growth," of " constitutional tendencies," or of the
precise chemical nature of the protoplasmic metabolism in the
organisms in question. If constitutional variations occur along
a few de6nite lines, as Eimer, Geddes, and others have main-
tained in certain cases, then we can understand the origin,
though not perhaps the distribution, of species apart from any
long process of selection, for which indeed, if variations be
strictly definite, the material must be vastly reduced. In
other words, we can think of the organism not merely under
the moulding influence of its functions, nor solely as the pro-
duct of environmental hammering, least of all as the survivor
from a crowd of unsuccessful competitors, but as the expression
of an internal fate, no longer mystical, but expressible in terms
of the dominant chemical constitution.
§ 2. The Reproductive Factor, — Without further discussion
of the still open controversy as to the various factors of evolu-
tion, which would not be relevant to such a work as this, we
must summarily collate the more prominent opinions as to the
share reproduction has in the process. To most of these we
have already alluded in the body of the book.
(a,) First of all, as to the origin of variations, we find that
what Treviranus recognised in the first years of this century —
viz., the influence of fertilisation in evoking change — has been
emphasised by several, such as Brooks and Galton, and has
been especially elaborated by Weismann. As we have already
noted, Weismann has suggested that the intermingling of two
"germ-plasmas," which is at least part of the essence of
fertilisation, may be an important fountain of congenital
variations. In apparent contrast is the view advocated by
Hatschek, who sees in the intermingling essential to fertilisa-
tion a counteractive of idiosyncrasies, a means of controlling
and checking disadvantageous individual peculiarities. The
two positions are not antagonistic, but rather complementary.
(^.) No impartial student of Darwinism can fail to admit,
that in the '* struggle for existence" stress is laid upon the
324 THE EVOLUTION OF SEX.
nutritive and self-maintaining functions and strivings, yet we
must remember Darwin's own words: — "I should premise
that I use this term [struggle for existence] in a large and
metaphorical sense, including dependence of one being on
another, and including (which is more important) not only the
life of the individual, but success in leaving progeny" ("Origin
of Species," p. 50). Similarly, Herbert Spencer says : " If we
define altruism as being all action which, in the normal course
of things, benefits others instead of benefiting self, then from
the dawn of life altruism has been no less essential than
egoism. Though primarily it is dependent on egoism, yet
secondarily egoism is dependent on it." " Self-sacrifice is no
less primordial than self-preservation" ("Principles of Ethics"
and " Principles of Psychology").
(c.) Darwin also insisted upon the rd/e of "sexual selection,"
which implies a recognition of the reproductive factor. We
have seen, however, that sexual selection is only a special case of
natural selection ; that it seeks to explain the elaboration, not
the origin of sexual peculiarities; and lastly, that Darwin's
arguments in favour of the mechanism which he emphasised,
have been seriously impugned by Wallace, in an attack which
reacts strongly upon the critic's own position. It must not be
overlooked, however, that the existence of any form of selective,
as opposed to indiscriminate mating, will, if natural selection
be at work, tend to accelerate the process of differentiation.
(See Pe<irson's "Grammar of Science," 2nd edition, 1900,
pp. 423-437)
{d.) Romanes has recently elaborated, what others seem
also to have suggested, the importance of mutual sterility in
splitting up one species into several. " Whenever any varia-
tion in the highly variable reproductive system occurs, tending
to sterility with the parent form without impairing fertility with
the varietal form, a physiological barrier must interpose,
dividing the species into two parts, free to develop distinct
histories, without mutual intercrossing, or by independent
variation." The reproductive system is very apt to vary, —
why, he does not say; the consequence might readily be,
that among the progeny of a parent slock some were fertile
inffr se, but infertile with the consistent members of the
parent stock ; these will be isolated by a physiological barrier,
just as they might be insulated by a geographical one, and
left free to develop along divergent paths of their own. Here
THE REPRODUCTIVE FACTOR IN EVOLUTION. 325
again there is recognition of the reproductive factor in evolu-
tion ; but how far, and in what cases species have so originated,
is obviously a question which would involve discussion of each
individual instance.
(e,) When pairing is restricted in its range to more or less
closely related forms — t\e., when inbreeding is practised — it
seems to have up to a given limit the effect of fixing characters
and developing prepotency. By prepotency is meant a relative
strength in the power of transmitting character, or, in other
words, the strong persisting power which certain characters
have over others in inheritance. Thus if a certain character
of a sire is invariably transmitted to the offspring, irrespective
of the character of the dam, we say that the male is prepotent
as regards that character. But if inbreeding occurs in nature
— ^^., as the result of some form of isolation, as it does in
domestication ; and if it has as one of its results the develop-
ment of prepotency, we have here an important factor in
evolution. For the prepotency will account for the persistence
of variations in their early stages. (See *'The Penycuik
Experiments," by Professor J. Cossar Ewart, London, 1899,
xciii. and 177 pp., 46 iigs.)
(/.) There is among organisms, and even in allied species,
an enormous variety in the actual fertility and in the potential
fertility (fecundity) ; even among members of the same species
there are great differences. The importance of this is increased
when we note that Professor Karl Pearson, with the assistance
of Miss Alice Lee and Mr. Leslie Bramley-Moore, have shown
that fertility is inherited in man and fecundity in the horse — a
conclusion which probably admits of great extension. The
fact gives basis to Pearson's theory of " reproductive or genetic
selection " — a phrase which he uses to describe the selection
of predominant types owing to the different degrees of repro-
ductivity being inherited, and without the influence of a
differential death-rate. If fertility is inherited, and if fertility
is correlated with other characters, there will be a continual
tendency to progress in a definite direction, unless there be
some counteracting factor which takes the form of a differential
death-rate more intense for the offspring of the more fertile.
("Proc Roy. Soc. London," Ixiv., 1899, pp. 163-167.)
(^.) Worthy of consideration is the suggestion of Robert
Chambers, crudely illustrated as it may have been, that
environmental influences acted with special power upon the
326 THE EVOLUTION OF SEX.
generative system, and that the prolongation of gestation was a
maternal sacrifice which brought its own reward in the higher
evolution of the offspring. With this Beard's interesting essay
on **The Span of Gestation" may be profitably compared.
Miss Buckley has well pointed out how the increase of parental
care was a factor in, as well as a result of, the general ascent;
how the success of birds and mammals especially must in part
be interpreted in reference to the noteworthy deepening of
parental affection, and strengthening of the organic and
emotional links between mother and offspring. In emphasis-
ing the progressive value of prolonged infancy, especially in
the evolution of the emotions, Fiske has also recognised the
importance of the reproductive factor. The same idea fs
prominent in Drummond's "Ascent of Man."
§ 3. Further Construction. — The general tendency of all
theories of evolution has been to start with the individual
organism as the unit, and to consider the self-maintaining and
nutritive activities as primary, the reproductive and species-
regarding as only secondary. But along many lines of
research, such as those indicated in the preceding paragraphs,
the importance of the reproductive factor has been recognised,
and the centre of gravity of the evolutionary inquiry has already
been to some degree shifted. Recent investigations on heredity,
for instance, forbid that attention should any longer be con-
centrated on the individual type, or reproduction regarded as a
mere repetition process; the living continuity of the species
is seen to be of more importance than the individualities of the
separate links. Physiologists and evolutionists are coming to
see the most complex individual lives, in Foster's phrase, as
" but the bye-play of ovum-bearing organisms." The species
is a continuous undying chain of unicellular reproductive
units, which indeed build out of and around themselves
transient multicellular bodies, but the processes of nutritive
differentiation, and other individual developments, are second-
ary, not primary.
Thus it is the central generalisation of botany that, despite
the individual differentiation of fern, selaginella, cycad, conifer,
and flower, these turn out, on deepest analysis, to be but the
surviving phases of a continuous and definite increase in the
subordination of the sexual parents to their asexual offspring.
Or if we take in particular the origin of the flower, which
many botanists agree in regarding as a shortened branch, the
rate REt>R6DUCtIVte ir ACTOR IN EVOLUtlON. ^IJ
natural selectionist, explanation would seem to be, that the
flower had arisen by selection from the two other alternatives
of lengthened and unshortened axes. But this point of view
must be corrected in relation to the physiological theory that
shortening of the axis was ineviiad/e, since the expense of the
reproductive functions necessarily checks the vegetative ones.
It is evident that we cannot speak of seieciion where the
imaginable alternatives are physically impossible. So too the
shortening of the inflorescence from raceme to spike or flower-
head, or still further into the hollowed form of a flg, with the
corresponding reduction in the size of the flowers, is again the
result of the check imposed by reproduction on the growth of
axis and appendages.
The same simple conception of a continuous checking of
vegetation by reproduction unlocks innumerable problems of
floral structure, large and small alike, from the inevitable
development of gymnosperm into angiosperm by the con-
tinuous subordination of the reproductive carpellary leaf, to
the variations of cabbages as seen in the transitions between
leafy kale and cauliflower. Or again, the origin of floral
colour, as primarily an inevitable consequence of the same
principle of vegetative subordination through reproductive
sacrifice, was long ago pointed out by Spencer, and admits of
detailed elaboration without attaching more than secondary
importance to selection by insects.
In another way, the antithesis between reproduction and
nutrition may be illustrated among the existing orders and
species of flowering plants. Just as the lilies, for instance,
range on the one side towards the characteristically vegetative
grass, or on the other towards the reproductive orchid, so it is
with the main variations of every natural alliance. Thus, the
Ranunculacese have their grassy and their orchid-like types in
meadow-rue and larkspur respectively, while the species of
these very genera show, within narrower limits, similar swings
of variation. What we call higher or lower species are thus the
leaders or the laggards along one or other of these two lines of
variation.
Among animals, the importance of the reproductive factor
may be illustrated in the most diverse series. Thus the
greatest step in organic nature, that between the single-celled
and many-celled animals, bridged as it is by loose colonies,
some of which are at a very low morphological level, is not
3jS tHE EVOLUTION OF SEX.
due to the selection of the more individuated and highly
adapted forms, but to the union of relatively unindividualed
cells into an aggregate, in which each becomes dimiiishingly
competitive and increasingly subordinated to the social whole.
The colonial or multicellular forms, originating pathol(^icalty
in all probability, may of course have rapidly justified their
existence in the struggle for existence, just as unions of many
kinds do in human society, but the Protozoa cannot be accused
of any prevision of future advantage in remaining clubbed
together in co-operation, nor indeed credited with much
primitive altruism in so doing.
No structure is more emphatically nutritive in its adult
result than the gut-cavity of the embryonic gastrula. It is
worth inquiring whether this important step in differentiation
was attained in history in response to nutritive needs. The
usual supposition is certainly that the gastrula cavity, by what-
ever peculiarities of giowth it may have arisen, justified itself
THE REPRODUCTIVE FACTOR IN EVOLUTION. 329
from the first in an additional nutritive advantage. But
Salensky, in his studies on the primitive form of the Metazoa,
has given strong arguments in favour of the theory that the
primitive cavity, arising in a volvox-like form, was originally a
brood-cavity or "genitocoel," and that it only secondarily acquired
nutritive significance. It would be indeed striking if this
important morphological step in the establishment of the
nutritive system was reached along the road of reproductive
modification; for if this most fundamental of nutritive and
self-maintaining advantages, the belly itself, be but a secondary
resultant of an originally reproductive and species-regarding
progress, the lower Utilitarianism, which has so long been
arguing from economics to biology and back again, is evidently
a step nearer exposure.
Or again, that increase of reproductive sacrifice, which at
once makes the mammal and marks its essential stages of
further progress through oviparous monotreme, prematurely-
bearing marsupial, and various grades of placental; that in-
crease of parental care; that frequent appearance of sociality
or co-operation, which even in its rudest forms so surely
secures the success of the species attaining it, be it mammal or
bird, insect or even worm, — all these phenomena of survival of
the truly fittest, through love, sacrifice, and co-operation, need
far other prominence than they could possibly receive on the
hypothesis of the essential progress of the species through
internecine struggle of its individuals at tlie margin of subsist-
ence Each of the greater steps of progress is in fact associated
with an increased measure of subordination of individual com-
petition to reproductive or social ends, and of interspecific
competition to co-operative association.
The corresponding progress in the historic and individual
world, from sex and family up to tribe or city, nation and race,
and ultimately to the conception of humanity itself, also
becomes increasingly apparent. Competition and survival of
the fittest are never wholly eliminated, but reappear on each
new plane to work out the predominance of the higher, r^.,
more integrated and associated type, the phalanx being
victorious till in turn it meets the legion. But this service no
longer compels us to regard these agencies as the essential
mechanism of progress, to the practical exclusion of the asso-
ciative factor upon which the victory depends, as economist
and biologist have too long misled each other into doing. For
330 THE EVOLUTION OF SEX.
we see that it is possible to interpret the ideals of ethical pro-
gress, through love and sociality, co-operation and sacrifice,
not as mere ulopias contradicted by experience, but as the
highest expressions of the central evolutionary process of the
natural world. The ideal of evolution is indeed an Eden;
and although competition can never be wholly eliminated,
and progress must thus approach without ever completely
reaching its ideal, it is much for our pure natural history to
recognise that " creation's final law " is not struggle but love.
The fuller working out of this thesis, however, would lead us
far beyond our present limits, towards a restatement of the
An Opojsum (.DiMfhyi dirtlgtra) carryinj iK young oo in back.—
From Caiiu Sume.
entire theory of organic evolution. Suffice it here, in con-
clusion, to indicate an important change in the general point of
view. The older biologists have been primarily anatomists,
analysing and comparing the form of the organism, separate
and dead; however incompletely, we have sought rather to be
physiologists, studying and iiUerpreting the highest and intensest
activity of things living. From the study of individual structure
they were wont to pass, indeed, to that of reproductive
structures, and thence even functions; hence, too, the pair
and the totality of the species did at length come successively
into view, but this with the individualistic theory of natural
THE REPRODUCTIVE FACTOR IN EVOLUTION. 33 1
selection bulking as practically all -important in the foreground.
For us, however, this perspective has become entirely reversed.
The individual is a mere link in the species, and its repro-
ductive processes are thus of fundamental importance to the
interpretation even of its self-maintaining ones. Hence we no
longer regard, with Darwin and the majority of our brother
naturalists, the operation of natural selection upon individual
characters as the simplest of problems, looking for residual
explanation to sexual selection, and only in extreme difficulty
invoking the aid of "principles of correlation," "laws of
growth," and the like, viewed as almost inscrutably mysterious.
On the contrary, it is the continual correlation yet antithesis —
the action and reaction — of vegetative and reproductive pro-
cesses in alternate preponderance, which seems to us of
fundamental importance, since to this the general rhythm of
individual and racial life runs fully parallel. Hence it is that
we have the primeval lily developing on the one hand the
ideally vegetative grass, yet also the supremely specialised re-
productive orchid; and that we can trace (as we hold) the same
swing of divergent evolution, of definite variation, in every
natural order, nay, in every genus, often even in the very
varieties of a species. Hence, too, it is that the rhythm of
hydroid and medusoid in the individual life of the typical
forms becomes fixed in coral or ctenophore as a racial tempera-
ment. This preponderance of passivity or activity (which we
can read throughout, in barnacle and insect, as well as in tortoise
and swallow) once set up, goes on accumulating till it meets
reversal through environment or other causes, and limitation
or extinction through the agency of natural selection, which,
however, is more frequently a retarding force than an accel-
erant of evolution. The problem of organic progress is thus
to be interpreted not ftierely as on conventional lines, by help of
an analogy derived from an age of niechanical progress which
gives us the watch, or sewing-machine, or tricycle, — by the
cumulative patenting, as it were, of useful improvements in
detail. The essential problem is not one of mechanism but
of character, to which incident is accessory but not fundamen-
tal, — not of details put together, but of aggregate organic life
or temperament. The life of the individual or the species is
essentially a unity, of which the specific characters are but
the symptoms, be their subsequent measure of importance
and utility in adaptation, their modification by environment.
33 a EVOLUTION or SEX.
their enhancement or diminution by natural selection, what
they may. Our special study of the reproductive process has
thus fairly brought us to the threshold of a larger inquiry, the
primary one of the organic sciences, that of the factors of
organic evolution. For it is in nature, as Schiller saw long ago
in the human life, which this foreshadows: *' While philo-
sophers are disputing about the government of the world,
Hunger and Love are performing the task."
4
THE REPRODUCTIVE FACTOR IN EVOLUTION. 333
SUMMARY.
1. A brief review of the history of evolution theories, and of the present
state of the question.
2. A reproductive factor in evolution has been hinted at by a few
naturalists.
3. Further indications of the importance in evolution of reproductive
and species-regarding, as opposed to the nutritive and self-maintaining
activities^
LITERATURE.
See articles by the writers in "Chambers's Encyclopjedia," especially
Biology, Botany, Environment, Evolution, and minor articles,
such as Ccelbnteratbs, Flower, Fruit, &c. ; also *' Encyclopaedia
Britannica," Sex, and Variation and Selrci ion ; also Grddes,
A Restatement of the Theory of Organic Evolution (Summary in
Proc. Roy. Soc. Edin., 1888-89, still unpublished).
Spencer, Mivart, Eimer, Wallace, Weismann, &c. — Op, cil.
Thomson, J. A.— The Endeavour after Well-being. Natural Science,
VIIL (1896), pp. 21 26.
INDEX.
-M-
Agr of parents, influence of, on sex,
37
Albrecht, parthenogenesis of silk-
moth, 183
Aldrovandus, 89
Algse, reproduction in, 135, 136
Allantois, 266
Alternation of generations, 213, 225
Alternation between sexual and
degenerate sexual reproduction,
216
Altruism, egoism and, 295, 296
Amici on pollen-grain, 148
Amnion, 266
Amphibians, parental care in, 270,
271
Anabolism, see Protoplasm
Angiostomum, 77
Animal cell, division of, 248
Animalculists, 89, 169
Anther, section of, 148
Antlers, 26; and reproductive
organs, 256
Aphides, 49 ; parthenogenesis in,
187 ; generations of, 189
Apospory, 219
Aquapendente, Fabricius ab, 89
Argonauta, 262
Anemia salina, 50
Artificial division, 200
Artificial parthenogenesis, 184
Arthropods, hermaphroditism in,
Ascaris, sperms of, 118; segrega-
tion of germ-cells in, 99 ; Prof.
Wilson's account of, 123
Asexual reproduction, 200-211 ; in
plants, 203
Asexual propagation of a grass, 20
** Aura seminalis," 170
Aurelia, alternation of generations
in, 215
Baer, Von, discovery of mammalian
ovum, 92
Balbiani on Chironomus, 98; conju-
gation in infusorians, 158
Balfour on polar bodies, 113;
theory of parthenogenesis, 194
Barfurth, parthenogenesis in hens'
eggs, 185
Barry, Martin, 150
Bary, De, 172; parthenogenesis
in fungi, 190
Beard, hermaphroditism in Myzos-
lorn a, 83 ; rhythm in vertebrates,
223, 264, 265
Bees, sex in, 46 ; origin of sex-cells,
97
Beneden, Van, polar globules, 113;
ovum of threadworm, 153, 155
Berner's statistics, 38
Berthold, 137
Bich^t's analysis, 92
Bidder's organ, 74
Bilharzia, 77
Bird of Paradise, 4
Birds, eggs of, no, in ; courtship
of, 6
Blackcock, 7
Blochmann, polar bodies, 112;
parthenogenetic ova, 194
Body and reproductive cells, rela-
tion between, loi, 278
Boerhaave, 89
Bonellia, 20, 21
33^
INDEX.
Bonnet, parthenogenesis in aphides,
183
Bordage, regeneration, 202
Born, hybridisation in amphibians,
163
Botrychium, 240
Boveri, researches, 153, 154
Brauer, parthenogenetic ova, 194
Brodie, Dr T. G., on spermatozoa,
124
Brooding and young-feeding organs,
69
Brooks, W. K., sexual selection,
11-13; variability of males, 127 ;
fertilisation, 178; alternation of
generations, 222
Bryophyllum calycinum with adven-
titious buds, 211
BUchner on love among animals,
Buckley, Miss, on parental care,
326
Buffon, 316
BUtschli on polar globules, 1 14
Butterflies and moths, gex in, 50
Callionymus lyra, 6
Camerarius, 148
Canestrini, 36
Carnoy on ferlilisntion, r53
Castration, 23, 24, 256
Casual hermaphroditism, 72
Cecidomyia, 258
Cell, animal, 104
Cell-cycle, 127, 130
Cell-division, 235, 273
Cell-theory, 92
Cells, body and reproductive, 99
Cephalopterus ornalus, 7
Chambers, Robert, on prolonged
gestation, 318
Chamisso, alternation of genera-
tions, 212, 213
Chemistry of the egg, in
Chironomus, 98 ; parthenogenesis
in, 188
Chondracanthus, 17
Chromatin elements, 254
Cladocera, seasonal parthenogenesis
in, 187
Claspers, 263
Coccus insects, 16
Coelebogyne, 190
Coelenterates, reproductive organs
in, 64, 65 ; hermaphroditism in,
76 ; asexual reproduction in, 205-
207 ; alternation of generations
in, 222
Coiter, Volcher, 89
Colour, animal, 25
Colour, floral, 327
Comet form of starfish, 210
Comparative vigour, theory of, 38
Complemental males, 82
Composite ova, no
Conjugation in Protozoa, 157-160;
multiple, 157
Continuity of germ-plasm, 100-102,
252
Copulation, 263
Corals, budding of, 208
Crustaceans, sex in, 50; partheno-
genesis in, 50
Cucumaria crocea, 292
Cupid's dart, 263
Culleria, reproduction in, 137
Cuttlefishes, reproductive organs of,
68
Cynipidae, parthenogenesis in, 187
Cyprids, reproduction in, 189
Daphnids, 189
Darwin, 5; sexual selection, 8-14;
and Wallace, 30; determination
of sex, 40 ; fertilisation of flowers,
146, 147 ; the population ques-
tion, 300, 306; natural selection,
3191 320
Darwin, Erasmus, 316, 317
Death, origin of, 248, 276, 278
Degenerate sexual reproduction,
183-198
Degrees of asexual reproduction, 203
Delage's experiments on fertilisa-
tion, 172
" Descent of Man," 5
Determination of sex, 34
Development, stages of, 91
Dewitz, 151
Dichogamy, 78
Didelphys dorsigera, 330
Diplozoon paradoxum, 262
Divergence of sex-cells, 128, 134
Division of labour, 2o6
INDEX.
337
Doliolum, alternation of generations
in, 216
Drelincourt, 130
Drones, 19
Ducts of reproductive organs, 66, 67
Dufour, parthenogenesis in aphides,
183
DUsing, 36; on proportions of
sexes, 40, 41
Dzierzon, parthenogenesis in bees,
184
Echinoderma, hermaphroditism
among. 77
Ectocarpus, reproduction in, 137
E<1ible birds' nests, 267
Sggi chemistry of, 1 1 1
Egg-cell, 104- 1 14
Egg envelopes no
Egg-laying organs, 6S
Egoism, 295, 296
Eimer on sex in bees, 47
Embryology, history of, 88
Embryonic hermaphroditism, 71
Encystation, 206, 276
Engelmann on dimorphism in Pro-
tozoa, 138
Environment, influence of, 249, 250
Epigamy, 209
Epigenesis, 88; Wolfi's reassertion
of, 91
Epistylis, 138
Ernst, Dr A., parthenogenesis in
plants, 190
Ethical aspects of reproduction, 283-
,297
Eunice viridis, reproduction in, 210
Eupagurus liernhardus, 21
'* Evolution*' in old sense, 90
Evolution, reproductive fiactor in,
316-332
Ewart, Prof. Cossar, experiments in
hybridisation, 164, 105
Factors in determination of sex, 54
Fallopian tube, 257
Females, 1-52 passim; size, 19;
{passivity, 18; preponderant ana-
x>lism of, 28 ; psychical char-
acters of, 286-291
Ferns and mosses, alternation of
generation in, 217-219
Fertilisation, time of, 36; in plants,
146; in animals, 150-157; pro*
cess of, 154, 155; origin of, 158-
162 ; contrasted with conjugation,
161; theory of, 169-180; a source
of variation, 317
Fishes, sex differences in, 25 ;
parental care in, 272-295
Flower, 239
Fluke, alternation of generations in,
216, 217
Follicular cells, 1 10
Free-martin, 41
Fulton, Dr W. T., on male and
female fishes, 22
Fungi, parthenogenesis in, 191
Gait-wasps, parthcnc^enesis in, 190
Galton on body and sex-cells, lOo;
on use of fertilisation, 170
Gastroblasta raffaelii, 206
Gastrula, 91, 328
Gegenbaur on origin of hermaphro-
ditism, 84
Gemmules of fresh-water sponge,
222
Genesis, 285
Genetic selection, 325
Genoblasts, theory of, 127
Gentry on sex in moths, 50
Geoffroy, E. F., 148
Germiparity, 71
Gestation, period of, 263; influence
of, prolonged, 318
Giard on polar bodies 1 14
Girou on comparative vigour, 3^;
on sex in sheep, 51
Glyciphagus cursor, 206
Goette on nemesis of reproduction,
272. 275
Gonads, or essential reproductive
organs, 63-67
Graaf, De, 89, 170
Growth, facts oif, 233; Spencer's
theory of, 234
Gruberon reproduction in Protozoa,
24»
Guaila, hybridisation in mice, 164
Gyrodactylus, 220
Haeckel on contrast between body
and reproductive cells, 90; on
22
338
INDEX.
Protomyxa, 129; on alternation
of generations, 226 ; on mago-
sph»ra, 272
Haller, 89, 90
Hamm, discovery of spermatozoon,
117
Hartogt nuclear reduction, 196
Harvey, 88, 171
Hatchett Jackson on union in in-
fusorians, 138
Hatschek on fertilisation, 179; on
nutrition and reproduction, 241
Heat, influence of, on reproduction,
249
Hectocotylus, 68, 162
Helix, reproductive system of, 68 ;
hermaphroditism in, 79
Hensen, theory of sex, 36, 38;
length of life of sperms, 120; use
of fertilisation, 175 ; partheno-
genesis in bees, 186
Heredity, see Weismann, in alter-
nating generations, 224, 225 ;
acquired characters, 321, 322
Hermaphroditism, 71-84; conditions
o^ 83; origin of, 84
Heme, J. van, 89
Hertwig on hermaphroditism, 74;
theory of fertilisation, 171
Heterodera schachtii, 17
Heterogamy, 219
Heterophagy, 172
Heyer on sex in plants, 52
Hofacker and Sadler's law, 38
Hoffman, 38
Human species, influence of nutri-
tion on sex of, 5 t
Hunger and love, 296, 297
Hybrids, variability of, 164
Hybridisation, 162-166
Hydatina, 20; sex in, 51
Hydractinia, division of labour in,
206
Hydroids, alternation of generations
in, 218, 219
Immortality, organic, 275
Inbreeding, 179, 180
Incubation, 268
Individuation, 30 J, 307
Infusorians, 158; reproductive
power of, 249
Initects, sex characters of, 5, ri, [6-
19; determination of sex in, 46-
50; hermaphroditism in, 75 ; par-
thenogenesis in, 189
Isophagy, 172
Iwanzoff on fertilisation in Echino-
derms, 173
Jager, continuity of germ proto-
plasm, 100
Janosik, oarthenogenesis in mam-
mals, 105
Jaworowski, parthenogenesis in
Chironomus, 188
Thering, von, 223
Joseph on fertilisation, 171
joukowsky, conjugation in in-
fusorians, 178
Juvenile parthenogenesis, 187
Karyogamy, 162
Kennel, Professor von, on antlers,
26
Kerner von Marilaun, partheno-
genesis in Antennaria alpina,
190
Kerville on hybridisation, 165
Kirby and Spence, parthenogenesis
in aphides, 183
Klebs, reproduction in Algae, 162,
244
Kolliker on origin of sperms, 118
KoLreuter, 146; hybridisation in
plants, 166
Kossel on Protamin, 124
Lactation, 266, 267
Lamarck, 317
Laulani^ on embryonic organs, 35 ;
hermaphroditism, 11, 22
Lecanium hesperidum, 20
Leeuwenhoek, 89 ; on spermatozoa,
117
Lessona on regeneration, T45
Leuckart, frog ova, 184, 185; alter-
nation of generations, 214
Limax, reproductive system of, 78
Limit of growth, 234
Liver-fluke, life-history of, 217
Liverwort, 241
Loeb on sea-urchins, 174, 185
Love, 284-294
INDEX.
339
Love of offspring, 291
Lubbock on queen ants, 120
Luciola, 27
Magosphsera, 272
Males, 1-52 passim; variability,
12, 13; activity, iS; size, 19;
preponderant katabolism, 28 ;
complemental males, 82 ; psychi-
cal characters of, 2S6-291
Male-cell or spermatozoon, 1 17-128
Male parthenogenesis in Algae, 191
Malpighi, 89
Malthus, theory of population, 300-
Mammals, weapons of, 7 ; sex in,
51 ; love among, 285
Mammalian ovum, 87
Mammary glands, 266, 267
Mantegazza, 13, 285
Marchal, Paul, 223
Mark on polar bodies, 1 1 5
Marsupials, 269
Maternal sacritice, 274-275
Mates, love of, 283-285
Maturation of ovum, 112
Maupas on conjugation and multi
plication in infusorians, 157, 158,
177, 242, 249, 250; use of ferti-
lisation, 175
Mayflies, 275
Mechanism of cell-division, 236
Meehan on sex in plants, 52 ; on
fertilisation, 80, 147
Menstruation, 259
Mesozoa, liberation of germs in,
273
Metazoa and Protozoa, 95
Metabolism, see Protoplasm
Micropyle, no
Microstomum, 208
Miescher on milt of salmon, T?4
Migration of sex-cells, 66
Milk, 267
Millington, 149
Minot on polar bodies, 113 ; theory
of genoblasts, 127-139 ; partheno-
genesis, 194
Mivart on sexual selection, 13
Mobius, 25 ; effects of vegetative
multiplication, 179 ; stickleback,
268
MoUiard, influence of temperature,
54
Molluscs, hermaphroditism among,
78
Moonwort, 240
Morgan, T. H , hybridisation in
echinodcrms, 164 ; regeneration,
202
Morphological theories of fertilisa-
tion, 170
Mosses, alternation of generations
in, 217-219
MUller, Johannes, 92
Multiplication, laws of, 299-314
Multiple conjugation, 160
Myrianida, 209
Myzostomata, 81
Nageli, origin of reproduction, 247
Natural selection, a check on diver-
gence of sexes, 31
Ncedham, 89; scope of, 319, 320,
331
Nematodes, alternation of genera-
tions in, 222
Nemesis of reproduction, 272
Neo-Malthusianism, 309-314
Nereis, reproduction in, 209
Normal adult hermaphroditism, 75
Notodelphys, 270
Nototrema, 270
Nucleus, behaviour of, in fertilisa-
tion, 154, 155; essential elements
of, 254
Nussbaum, continuity of germ-
plasm, 100
Nutrition, influence of, on sex, 45
Nutrition of embryo, 265
Nutrition and reproduction, 238,
295
Obstetric frog, 270
Occasional parthenogenesis, 186
Oellacher, parthenogenesis in hens'
eggs, 185
Ofl'spring, love of, 291
Offspring of parthenogenesis, 191-
192
Oken, 318
Onion with bulbils, 303
Ophrydium, 95
340
mbEk.
Organs auxilixiry to impregnation,
67,68
Organic immortality, 275
Orgyia, 5
Orlhoneclids, 274
Oudemans, J. Th., on castration, 24
Oviparous animals, 264
Oviparous mammaJs, 264
Ovipositors, 68
Ovists, 89, 169
Ovo- viviparous animals, 264
Ovulation, 257
Ovum or egg'cell, 104- 114; matura-
tion of, 112; liberation of, 257
Ovum theory, 88
Owen, alternation of generations,
213 ; on residual spermatic force,
224, 225
Psedogenesis or juvenile partheno-
genesis, 187
Pairing, early forms of, 284
Paper nautilus, brood shell of fe-
male, 271
Paramsecium, conjugation of, 158
Parental care, 291-295
Parthenogenesis, 183 194
Parthenogenetic ova, peculiarity of,
193
Partial hermaphroditism, 72
Parturition, 263
Pearson, Karl, forms of sexual re-
Lition, 261 ; variability, 290;
genetic selection, 325
Peckham on spiders, J 4
Penis, 263
Penycuik experiments, 165
Perez, parthenogenesis in silk-
moths, 185
Pfluger on jiolyspcrmy, 36, 45
Physiological theories of fertilisa-
tion, 172
Pieri, 174
** Pigeon's milk," 267
}*igmcnts as waste products, 25
Placenta, 264
Planarians, 238
Planta, A. von, diet of bees, 47
Plants, determination of sex in, 52 ;
conjugation in, 160, 161 ; fertili-
sation, 146-150; parthenc^enesis
in, 190; alternation of genera-
tions in, 223 ; asexual reproduc*
tion of, 203 ; origin of sex among,
Plasmodium, 159
Plastogamy, 160, 162
Platner on parthenogenetic ova, 194
Ploss on embryonic hermaphrodi-
tism, 35
Polar globules, 112, 113
Pollen grain, 243
Polyspermy, 195
Polyzoa, budding in, 208
Population question, summary of,
305 ; development of theory of,
306
Precocious reproduction, 258, 259
Preformation theory, 89
Prepotency, 325
Provost and Dumas, 92
Primary sexual characters, 3
Protamine, 124
Proterospongia, 94
Protococcus, reproduction in, 136
Protomyxa, 129
Protoplasm, metalmlism of, 92-94,
131, 132, 237, 238; infertilisation,
171, 172; in cell-division, 235,
236
Protoplasmic movement, the, 93
Protozoa contrasted with meiazoa,
95, 96; illustrating cell-phases,
I32» '33 » fertilisation in, 157,
158; conjugation in, 162; im-
mortality of, 275-280 ; alternation
of generations in, 221
Psychidse, sex differences in, 27
Puberty, 255
Purkinjc, 92
Pycnogonid, male carrying the
young, 293
Kate of increase, 299
Rate of reproduction, 299, 300
Rath, Dr O. vom, on tclegony,
166 ; experiments in breeding,
180
Rauber on body and reproductive
cells, 100
Reaumur on aphides, 183
Regeneration, 201, 202
Reibmayr, inbreeding, 180
INDEX.
341
Reproduction, different modes of,
145; sexual, 145-166; growth
and, 233, 244; theory of, 245-
251 ; in relation to environment,
249 251 ; nemesis of, 272-275
Reproductive cells, 87-124 passim;
V. somatic cells, 99, 100, 278-
280
Reptiles, amatory emotions of, 284,
285
Rhumbler, evolution of fertilisation
processes, 162
Rhythms of life, 238, 239
Richarz, 39
Rolph on antlers, 26 ; sex in wasps,
48 ; theory of sex, 1 30 ; theory of
fertilisation, 172, 173 ; partheno-
geneticova, 192, 195
Romanes, G. J., 14c ; on physio-
logical selection, 324
Rorig on castration, 24 : antlers and
reproductive organs, 256
Rotifers, parthenogenesis in, 187 ;
male, 275
Sabatier, theory of polarities, 114
Sachs, origin of fertilisation, 161,
162, 172
Sadler on determination of sex, 38
Salensky on primitive metazoa, 329
Salmon, 259
Salpa, alternation of generations in,
216
Schaffer, parthenogenesis in crusta-
ceans, 185
Schaudinn on reduction division,
"3
Schenk's Iheorj*, 52
Schiz(^aniy, 209
Schlechtcr on sex in horses, 54
Schleiden, 92, f4S
Schultze, two kinds of ova, 36
Schwann, 92
Sea-anemones, asexual multiplica-
tion in, 207
Sea-horse, parental care in male,
271, 272
Seasonal parthenogenesis, 187
Secondary sexual characters, 3-8,
16-27
Self-fertilisation, 79
Sellheim on castration, 24, 256
Seminal vesicles, 255
Sex, determination of, 34-57 ;
theory of, 130140; origin of,
135; special physiology of, 253-
282
Sexes, differences between the, in
general habit, 16-19; in size, 19;
in other characters, 21 ; intel-
lectual and emotional differences,
286
Sex -elements, the ultimate, 87-102,
general origin of, 97; early separa-
tion of, 98
Sexual attraction, 285
Sexual diathesis, 23
Sexual maturation, 255
Sexual organs and tissues, 63-69
Sexual reproduction, 145-158
Sexual selection, 8-14 ; its limits as
an explanation, 27
Sexual union, 245 247
Shufeldt on sex in birds, 36
Siebold, Von, experiments on
wasps, 48; on sperms, 118;
parthenogenesis, 184, 186, 188
Silkmoth, parthenogenesis in, 185
Silvestri on sperms, 1 19
Simon, origin of sex, 173; par-
thenogenetic ova, 193
Siphonophore colony, 207
Skin-outgrowths, 26
Snail, reproductive specialisation
in, 68
Spallanzani on fertilisation, 170
Spencer, theory of growth, 234 ;
laws of multiplication, 30J ;
population question, 299 ; factors
m evolution, 320
Spermatogenesis, 122
Spermatic animalcule, i iS
Spermatozoon, 1 1 7- 1 28 ; discovery
of, 117; structure of, I18-120;
physiology, 120; contrasted with
ovum, T23; influence of, 317
Spermatozoa of crayfish, 119
Spiculum amoris, 263
Spiegelberg, 4 1
Spirogyra, conjugation in, 160
Sponge, hermaphroditism in, 76 ;
colony of, 201 ; asexual multipli-
cation in, 205 ; alternation of
generations in Spongilla, 222
342
INDEX.
Spores, 219
Sprengel on fertilisation of flowers,
146
Standfuss on hybridisation, 165
Starfish, regrowth of lost parts, 210
Starkweather's law of sex, 39
Statistics on male and female births,
37
Steenstrup, alternation of genera-
tions, 213
Steno, 89
Sterility, 314
Stickleback, 6 ; nest-building of,
25 ; secretion of kidneys, 2^
Stieda's statistics, 38
Stratz, 260
Strasburger on fertilisation of plants,
149, 171 : parthenogenesis in
plants, 190, 195
Suchetet on hybridism, 163
Summer eggs, 189
Surinam toad, 269
Sutton on embryonic hermaphrodi-
tism, 35. 54
Swammerdam, 89
Syllis ramosa, 210
Syllids, alternation of generations
in, 216
Tadpoles, sex in, 45
Tapeworm, 222
Telegony, 166
Temperance, 313
Temperature, influence of, on sex,
5U 53
TichomirofF, parthenogenesis in
silkmoths, 184
Tiger lily, 240
Thomson, Allen, 89
Thury, experiments in breeding, 36
Treviranus, fertilisation as a source
of variation, 17S; influence of
spermatozoon, 317
Tunicates, asexual multiplication,
211 ; alternation of generations,
223
Tutt, J. W., hybridisation, 165
Twins, sex of, 4 1
Ulothrix, reproductive cells of, 137
Variation, 13, 178, I79» 322, 323
Vernon, hybridisation in echino>
derms, 37, 163, 165
Vines on reproduction in plants,
136, 137
Volvox, 64, 137-139. 161
Vorticella, 138; conjugation in, 158
Wagner, 92; juvenile partheno-
genesis in midges, 187
Wallace, sexual selection, 10, 11 ;
natural selection, 320
Wallengren, conjugation in protozoa,
157 ^ .
Ward, Marshall, parasitic fungi,
243
Warneck, 150
Waterfleas, parthenogenesis in, 189
Weeping willows, 200
Weismann, fertilisation, 173, 175,
176; polar bodies, 112, 114
theory of parthenogenesis, 195
196; origin of variations, 179
192, 193, 322; Daphnids, 189
hydroids, 224 ; continuity of germ
plasm, 100-102, 253 ; regenera
tion, 202 ; alternation of genera
tions, 227 ; organic immortality
276 ; reproductive and body
cells, 279; inheritance of acquired
characters, 321-323
Whitman on polar bodies, 114
Wilson, £. B., 99; Ascaris, 123;
fertilisation, 157
Wind-eggs, no
Winter eggs, 189
Witches' milk, 267
Wolff, reassertion of epigenesis, 91,
92
Women, 286-291
Worms, hermaphroditism in, 76;
asexual reproduction in, 208 ;
alternation of generations in, 222
Yolk glands, 67
Yolk, 107
Yung on sex in tadpoles, 45
Zacharias on sperms, 119, 124
Zona pellucida, 1 10
Zona radiata, 1 10
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((
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XXVL APPARITIONS AND THOUGHT - TRANSFER-
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Industry among Primitive Peoples. By Otis T. Mason,
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National Museum.
"A valuable history of the development of the inventive faculty." —
Nature,
XXIX. THE GROWTH OF THE BRAIN : A Study of
the Nervous System in relation to Education. By
Henry Herbert Donaldson, Professor of Neurology in the
University of Chicago.
** We can say with confidence that Professor Donaldson has executed his
work with much care, judgment, and discrimination." — The Lancet,
New York : Charles Scribnkr*s Sons.
XXX. EVOLUTION IN ART: As Illustrated by the
Life-Histories of Designs. By Professor Alfred C
H ADDON. With 130 Illustrations.
*' It is impossible to speak too highly of this most unassuming and
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XXXL THE PSYCHOLOGY OF THE EMOTIONS. By
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Revue Philosophique,
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Academy,
XXXII. HALLUCINATIONS AND ILLUSIONS : A Study
OF THE Fallacies of Perception. By Edmund Parish.
**This remarkable little volume." — Daily News,
XXXIIL THE NEW PSYCHOLOGY. By E. W. Scripture,
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XXXIV. SLEEP : Its Physiology, Pathology, Hygiene, and
Psychology, By Marie de ManaceTne (St. PetersbvrgX
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XXXV. THE NATURAL HISTORY OF DIGESTION.
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Ed. With a large number of Illustrations and Diagrams.
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prehensive collation of this kind exists in recent English Literature." —
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By Professor EuciENE S. Talbot, M.D., Chicago. With
Illustrations.
**The auilior is bold, original, and suggestive, and his work is a con-
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XXXVII. THE RACES OF MiVN : A Sketch of Ethno-
graphy AND Anthropology. By J. Deniker. With 17S
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** Dr. Deniker has achieved a success which is well-nigh phenomenaU" —
British Medical founial.
XXXVIII. THE PSYCHOLOGY OF RELIGION. An
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Professor of Education, Leland Stanford Junior University.
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New York : Chaki.es Scribner's Sons.
XXXIX. THE CHILD : A Study in the Evolution of Man.
By Dr. Alexander Francis Chamberlain, M.A., Ph.D.,
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(Mass.). With Illustrations.
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XL THE MEDITERRANEAN RACE. By Professor Sergi.
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XLL THE STUDY OF RELIGION. By Morris Jastrow,
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admirable introduction to any particular branch of it." — MeihoJist Times.
XLII. HISTORY OF GEOLOGY AND PALAEONTOLOGY
TO THE END OF THE NINETEENTH CEN'lURY.
By Karl von Zittel.
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XLIIL THE MAKING OF CITIZENS : A Study in Com^
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B.Sc. (Lond.).
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XLIV. MORALS: A Treatise on the Psycho-Sociological
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to bear on his subject." — Education.
XLV. A STUDY OF RECENT EARTHQUAKES. By
Charles Davison, D.Sc, F.G.S. With Illustrations.
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^ XLVL MODERN ORGANIC CHEMISTRY. By Dr. ^
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^ and "THE WILD DUCK." With an Introductory Note.
a ^OL. IIL "LADY INGER OF OSTRAT," "THE yiKINGS
U AT HELGELAND," "THE PRETENDERS." With an
^ Introductory Note*
^ Vou lY. "EMPEROR AND GALILEAN." With an
Introductory Note by William Archer.
^ Vol. V. " ROSMERSHOLM," "THE LADY FROM THE
PQ SEA," **HEDDA GABLER." Translated by William
Archer. With an Introductory Note.
X /OL. VL "PEER GYNT: A DRAMATIC POEM."
^ Authorised Translation by William and Charles Archer.
Eh The sequence of the plays in each volume is chronological ; the complete
set of volumes comprising the dramas thus presents them in chronological
S order.
P *' The art of prose translation does not perhaps enjoy a very high literary
status in En^^land, but we have no hesitation in numbering the present
^ version of Ibsen, so far as it has gone (Vols. I. and II.), among the very
best achievements, in that kind, of our generation." — Academy,
*'We have seldom, if ever, met with a translation so absolutely
MIomatic." — Glasgow Herald,
New York: Chaklbs Scribnkr's Sons.
COUNTWAY UBRARY
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